Woah, I'm 12 and MtF too~!
I came out a little while ago and I'm already on the waitlist for a transgender youth clinic. I'm pretty new to Susan's, but from what I've seen, it's an amazing place to be and really helpful! Welcome!
My parents are amazing, and I have nothing but hope that yours will be too. Supportive parents are the best!!!
Been using Steam for a good 2 1/2 years and all of my friends have been super duper supportive too. Steam is a great place for trans* kids, as most (and I emphasize most) people are welcomed and supported through their journey. I think changing my "Real Name" on Steam was the best thing I've ever done in my entire life. Other than coming out. You should totally add me if you wanna PM me your account or w/e.
Anyway, before I transitioned, one of my best female friends allowed me to borrow some of her clothes and I often wore them when my parents weren't home. That really helped. All my life I've had a rather long and feminine hairstyle so that helped too. But what's probably helped the most is just the pronoun thing. I asked for my parents and friends to call me by "Sarah" and refer to me with female pronouns not too long ago, and that's been a huge help.
Honestly, though, coming out felt so... good... And it's something you should at least start preparing to do. Writing a note, or a text or something is probably the best way to do it. That way you can get all your thoughts out before getting interrupted, and they have time to react and think about it before they respond. Either way, good luck, hope you have a good experience and supportive parents.
Like I said, feel free to give me your steam profile and I'll toss you an add; wouldn't mind having a few more friends in the same boat as me. Unless you're the same gal' I saw on >-bleeped-<... actually come to think of it you probably are.