1. Not everyone is going to like your voice. Some people don't naturaly have a good voice, but they take singing seriously and practise correct technqiues and work their way up, until they do figure out how to get different tones and sounRAB from their voice. If you're serious about it, don't give up and keep practising. Post some samples of your singing and we'll give you tips on how to improove it.
2. Your vocal chorRAB grow more as you go through puberty. This does however, affect your voice and your ability to reach higher notes. If i'm correct, higher notes usually mean smaller vocal chorRAB, which children have. As you reach puberty, your vocal chorRAB are growing bigger, making them more deep. Don't loose sleep over that, keep working on your range, but remember, range isn't ALL THAT important. Also with high notes, make sure you back off the pressure and relax your throat. If i remember correctly, higher notes require less pressure and air. Just keep your throat relaxed.
3. Yes, mucous does affect your singing ability. It stops the vocal chorRAB from viabrating clearly and as a result, many people try to push harder to get a better tone. This only damages their voice. Avoid dairy products, soft drink, citrus to minimise mucous.
Best way for us to see if you're doing it right or wrong is to post some audio samples. The thing with screaming/rasping is, tone does actually come from the throat. You can't get "tone" from your stomach. Your stomach is just your damn stomach. You use the stomach to support your voice so that minimal tension is placed in your throat. Figuring out which muscles in your throat do what is essential to learning how to rasp/scream. If you try to do it all from the gut, you're going to get next to no tone at all. If you try to do it from all the throat, you'll be literally squeezing and forcing the sound out. You need to move your throat back a bit so it scrapes the air coming up from your diaphram, sending air into your soft pallete. That's how you're going to get a rasp tone. Post a sample, we'll give you some feedback.
I hope I got my info right and my advice helps a little. I am by no means a professional but I am continually working on my voice. Most of the stuff I learn is from reading over and over again Merkaba's posts and practising it.