No offense, but as a 12 year old you're not going to get far with this. I'm 15, have two jobs (and a third this summer) and barely have enough money to scrap together enough for a build. Besides, putting together a computer isn't as easy as some people make it seem: There is a certain degree of technical prowess involved.
Besides, by my calculations it would take 26 weeks to scrap together a meager build (around $600) and even 18 weeks to get together enough money to buy a respectable, but not good for gaming Inspiron or something like that from Dell. This is of course assuming you're going to recycle everything except the tower.
To get anything decent for gaming at high resolution and higher settings, you'd have to wait a year on your budget. This also means not buying anything. For a year.