Coming out soon...

My advice is always be honest with them. Speak from the heart and take the conversation very seriously. Talk about how you feel, what you know and answer the questions they might have as best you can. It is never easy to be brave and courageous.

What do you want to say to them?

Hopefully they will be supportive, but if not do you know how they will react? I started my friends first then told my family because at that point in my life my friends were more involved in my day to day life than my parents were..

 

I think he is right dude, plus i think you shouldnt try to anticipate their reaction, if your doing that it would lead to overthinking

 

A couple of things...
I'm getting the vibe that you're pretty sure that they'll be fine with it.
Plus if you already know you're gay at 13, then your mum will probably suspect/expect it too.
If you're comfortable with your sexuality at such a young age, she has raised you well!

Most people (let alone family) will not make fun of you when you come out.
Depending on their education, they might think it's a "faze" or "you're not old enough to know" or "you're confused", it seems very unlikely in your situation though. Couldn't hurt to be prepared with answers though.
Write down any questions that they're likely to ask and have answers for them before you take the plunge.

I agree with Hyaline and Peacemaker as well of course.

Peace be with you and good luck.