Nobody's voice is really bad, but it can be somewhat raw. After you have followed some of these voice exercises, you should be able to sing more comfortably without feeling ashamed of your voice.
The more nervous you are when you are singing, the more unstable and shaky your voice will sound. You need to simply relax. Take some deep breaths in and out, and ensure that you are as comfortable as you can possibly be prior to singing.
As opposed to singing through your nose or your throat, try singing and breathing through your stomach or your diaphragm. You can practice this by lying on the floor with the lights off and a book on your stomach. You're doing it right if while you are breathing in and out the book rises and falls when you breathe in and out. Then you should do the same thing while you are standing up. Singing and breathing from your diaphragm will increase the volume and stability of your voice although it may make you look somewhat chubby while you are inhaling through your stomach.
Sing a few bars or a scale of a song with your hands pressed somewhat lightly right underneath your ribcage on your stomach. You aren't breathing through you diaphragm if your voice remains stable while you sing a few notes and then push your hands sharply into your stomach. If you stomach makes a weird noise or shakes when you push, then you can continue.
If you are a boy sing a high scale in C-minor an occasionally jump from one note to an even higher one. Your voice shouldn't swoop and if it does you need to do something about it since it mot supposed to swoop. If you are a girl, do the same thing but in C-major and go higher inside of higher.