I'm going to move this over to Family, Friends and Relationships, where there's some more dating advice. Hmm, my advice is to keep it fairly brief but friendly. Talk about your hobbies. Put yourself in the mind of the person who's reading it. It's hard to start a conversation with someone when you know nothing about them. If you list a hobby, they might say "oh, I'm into horror movies too - which are your favourites?". It's a nice icebreaker! Remember, with online dating, it's always good to keep your wits about you. There are nice people out there, but they are hidden amongst some very strange people. Don't be too disheartened if it doesn't work for you straight away.
What Spartan said is pretty sound advice. To add onto that, make sure you are crystal clear about what it is that you're looking for. There are a lot of nutcases out there that will bombard you with really weird sh*t, but showing that you do have standards as to what it is you're tying to achieve will help to somewhat lessen having to deal with such people.