Too Skinny!

I'm thirteen years old, and have been interested in gaining weight for the last few years. I'm only about 90 lbs :( But I want to fatten up my belly, thighs and butt.
I don't think anyone I know will be very supportive if I suddenly balloon up, so I need a slow way of gaining weight, so know one really notices at first, and will be used to it soon!

My goal weight is 170 lbs in the next few years. If anyone has any tips, pllllleeeeaaassseee respond! :)

P.s. I'm so glad that there are other people like me that are will to share their feelings, this group ROCKS!!