Dressing the part

Uggs, skinny jeans, tshirt that would show more chest area than typical male stuff(don't go for too much eye catching color like neon pink or violet, use dark, gray or pastel colors, colorful paints on tshirts are ok tho) and buttoning up sweater or jacket(don't go for too tight) and few accesories if you'd like, something like necklace or scarf and bracelet.
All in all try not to look overcrowded or too colorful with the things you chose to wear.

---------- Post added 28th Jan 2012 at 01:55 AM ----------

Or use black not sheer tights instead of jeans if you'll feel comfortable with ant the tshirt is long enouh to cover butt and croch areas

 

I'm honestly not trying to sound like a smart-ass here, but just wear clothes made for women. There isn't as much a gender dichotomy in fashion as there used to be, and for the past several years tailored, menswear and "boyfriend" styles have been pretty fashionable. Chances are that with styles you're wearing now (still dressing as male), there are feminine - and generally more fashionable - equivalents made for women, especially if you're the t-shirt and jeans type.
If you don't know how to size women's clothing, a tailor's measuring tape (to take your own measurements - you can get 'em anywhere sewing supplies are sold) and an internet search on sizing women's clothing should do the trick. There are even sites for transgendered women that have detailed information for sizing clothing, even fitting bras should you happen to be at that point.
Good places to look for clothing for women your age are in the teen girls/young women's departments at discount and department stores (Kohl's, J. C. Penny's, Sears, Macy's etc.) or stores catering to your target demographic (American Eagle, Buckle, Aeropostale, PacSun and the like). These are just suggestions based on what young women in my area (NTX) generally seem to wear, but wear whatever kind of young women's clothing, apparel and accessories you like once you figure out what size(s) you are.

I know you didn't ask about this, but it's important: If you haven't started tucking already, you've pretty much got to when you start dressing as a woman. Though this seems obvious, unlike men's clothing, women's clothing isn't cut with a penis in mind, so the bulge of an untucked penis can end up being glaringly obvious, and hiding the bulge might end up looking just as conspicuous as the bulge itself. If you aren't tucking already, a search on the subject of tucking comfortably will show you how.
Other things to create a more feminine-looking body pre-HRT/-surgery (padding, prosthetic breasts, girdles/corsets) can help, but aren't as necessary as tucking to wear women's clothing since cisgendered women can be flat-chested and have straight ("boyish") figures.
[Also, for future reference, make sure to invest in quality padding and prosthetic breasts once you get to that point {if you get there before HRT and surgery}; the cheap foam stuff isn't worth your money!]

Hope I was of some help!

 

If you want to wear dresses and skirts, go for it! Seriously, don't let what other people are doing around you dictate what you should and shouldn't do. I sometimes don't want to leave the house because what I'm wearing is too "out there"--since no one else dresses the way I do--but if you think about it, that's a good thing. I have a unique sense of style and I know what looks good on me; I don't just go for the safe, easy tshirt and skinnies and Uggs combo. Not that there's anything wrong with that--but honestly, I prefer skirts and dresses. I like feeling super feminine. Try skirts and dresses out if you want to, seriously.

Another tip--accessorize, accessorize, accessorize! Accessories are your best friend and can help to change up outfits. Even something as simple as dangly earrings can just make your whole look more polished. Necklaces, bracelets, scarves... all are good. And, of course, rings! Rings are my favorite accessory, personally, and I love necklaces. Statement necklaces are the best way to change up outfits that can get boring (I tend to wear the same skirt/shirt combo sometimes when I get lazy and haven't gone shopping in a while, haha.) A good place to find accessories would be Forever 21--they have the best accessories, and they're pretty cheap. I mean, they're not the best quality, but it's good for trendy jewelry. Also, H&M. H&M is pretty much my favorite store for clothes and accessories.

 

Have you heard term "passing"?
Sure dress might make feel more femy ,but I don't think it's a good choice for m2f to go out for the first time as female especially if going out for simple walk out side, clubbing would be different thing.
Dress would attract much attention, thus lowering chance to pass and passing is essential to build up confidence.
Dresses will work once other passing related things gets polished.
And overaccesoriseing is a bad thing, as example more than few braces will make hands appear longer than they are.
Just explaining my previous post.

 

I have found that girls that age, will actually wear clothes at the very least, that are similar to dresses, and if they're going out with friends or on a date maybe, then they might wear more accessories, cheshire is very right in that you should definitely think about what helps you pass and what hinders that, try your outfits on at home and look in the mirror, if you have one that lets you see everything on you even better. Though when it comes to what is common, just look around at stores like what PoiYote mentioned, they have a pretty wide selection, look and see what you might consider to be pretty casual, something you might look and see on the street and think "oh, that's cute" but then not give it much more attention than that, just to help you figure out what might work for you, if you go out a lot where you're seeing other girls your age, not even actually knowing them, but at least seeing them, then you likely know a lot more than you might think, you can also try googling just that, pick a fashion niche you would like to use, and then do an image search of that, look around at what you like, you'll quickly learn what you like and don't like, on top of that, clothes will do different things to sort of, visibly re-shaping your body almost, it doesn't actually, but it looks like it, like cheshire mentioned, too many bracelets might make your hands look longer, however using accessories in places that are passing a little better than some other places is a good idea, also scarves, they can sometimes help to make your shoulders appear smaller as well as your neck, but make sure that that's what they're doing, try things on, look and see what they do, transsexuals are sort of lucky, GG's do still need to think about all of this stuff, and the ones that actually do, are the ones you might walk passed on the street and think to yourself "ooh, she's pretty" sorry if this is an old post, I just found it, and wanted to help anyone else who might be in a similar situation, and this is something that i'm very knowledgable on an for once felt comfortable speaking about. Also, an examples of outfits with skirts that aren't too showy or flashy.
this is a little bit too eye catching, bit this was a quick search and it does show that it is possible to wear a cute skirt casually walking around, in say the mall, or maybe going to a resteraunt, otherwise, it tends to be more comfortable shorts or skinny jeans, but those are still really 'femmy' feeling. lastly, nothing is right for everyone, I can't stress enough look in the mirror with your outfit on first, that way you won't be walking around with a bunch of people staring at you for the wrong reasons, you might even attract some good attention, though that is a "might." sorry for the long post on an old thread.

 

My advice is, wear what your comfortable in, you do have a fashion sense, it's been inside of you all of your life but you have learned to bury it. You'll figure it out, just listen to your thoughts. As for me, I'm not into dresses or skirts, I like really cute jeans, shorts and tops. Lots of cute ones out there, you just have to look.

P.S. As for tucking, I tried it and failed every time!! Also one word to sum up tucking, OUCH!!!
Jamie Lynn

 

I don't see how a skirt could hinder passing? I mean, I could see how a strappy dress might, especially a clingy one, because it would probably frame your physique the wrong way early in transition (showing off broad shoulders, skirting past slimmer hips and thighs), but I don't think skirts are impossible. Especially ones with ruffles or pleats, because they'd add dimention to hips and thighs and make them look more fem. If you paired one that nips in at the waist with the right top, I think you'd get a very feminine figure indeed. Also, color theory - dark colors tend to visually shrink an area, and light colors visually widen. So you might consider looking for cinch belts in darker colors to help you appear to have a slimmer waist. These are very easy to find, and girls tend to wear them with higher waisted skirts, dresses, or longer, tunic tops with leggings or skinny jeans. And as for casual dresses, I find it's harder to find them, especially during the colder months. But if you find a dress and you love what it does for you, wear it! Stack on whatever accessories you're comfortable with. Personally, I like the look of a few interesting pieces at a time - a neclace and couple of rings, for example - as opposed to lots of layers, but that choice is entirely yours.

Honestly, just wear what makes you comfortable. If you want to wear skirts and dresses, then do it. Look at different styles and colors, try things on, and see what looks the most flattering. Along with tips for transgirls specifically, you might look at fashion tips for a "ruler" figure - they show you how to build illusional curves. I hope this helps.