I'm gonna give you some tips:
1) DO NOT read the lyrics when you are trying to rap. Memorize them, feel them. See, first thing you should do is pick your musical reference/ favorite rapper. Now, pick one of his songs, one that you really feel, one that you usually sing along, one that makes you think "oh, I wanna be this great". Now, since you have an iPod (I had one too :c) which not everbody can afford, play that song, and sing to yourself, low, or even without making any noise. When you have done this like, IDK, 15 times, you'll already feel the track. Then start rapping along. Do these several times. You'll end up memorizing the most of it. For me it was harder because although I'm not a native speaker, I had a hard time till I started memorizing my stuff (I was damn young anyway). Now, by memorizing and FEELING what you are saying, you'll be WAY MORE CONFIDENT. Believe me. Also, subconsciously, you'll be adapting to your favorite artist's way of rapping.
2) I didn't feel really confortable with my voice, and I still ain't, BUT you'll get a lil more confortable with time. Then you can train your Voice, I didn't ever do this, but you can get someone to teach you. Also, as you mature your voice will get better. For adolescents, your voice is changing, thus when you are 20 already it will be way better than it was
3) Train your breathing. Make sure you are not one lazy moth******r! If you are lazy, or don't have any energy, you'll loose passion. It's not about being a fast rapper, but having a serious attitude towards it. Enlarge your lungs bro, run a lot, and even sing along your favorite songs while you run. Try singing along faster rap songs - let me remind you, memorize the lyrics or else you won't have time to read them and spit them, learn the rappers pace.
I hope that I was helpful.
All of this came from my experience
Peace.