Last time I was on here, I was horrified about coming out as a trans male, and I decided the best thing to do was to write my sister a letter, because I couldn't express a lot of my feelings and I thought she would have been supportive of me. A lot of screaming, yelling, and crying ensued, and it was a horrible mess. It ended up with me apologizing, and saying that "I'll stop." because I knew she wouldn't leave me be without making sure I "knew I was a girl." I have, however, in the last 8 months, I convinced my mother to let me cut my hair much shorter, and allowed to shop in the "boys" area. I'm still scared about trying again, and this time come out to my mother and sister, now that its been a while. I'm scared shitless, needless to say, and at least I know what to actually write down instead of some halfassed letter. I'm scared of what might happen, but I hope it'll work out.
Wishing all the best! Try and get all the support you can. If there is a local trans or LGBT community contact them. Remember you have support here.