First of all you have to relax. If you are tense it will sound through in your voice. Also being tense leads to have a tight muscles which lead to being flat or sharp bout things you want to avoid while you sing. Take a couple of deep breathes and don't force anything ever!
Posture. You must stand straight, no slouching. This affects your lung capacity. If you slouch you aren't using your entire lung capacity because you are incapable of doing it thus affecting your singing. Also you want to breath and fill your lungs, to do this push out your stomach when you are taking a breathe. This uses the entire lung area and allows your natural tone to come out while singing. This also allows you to uses your diaphragm to control the tone while you sing.
Stand with your feet about shoulder width a part in a comfortable position. Also have your arms slightly flexed when you sing. This allows your ribcage to move freely, allowing your lungs to have full capacity while you sing (notice a theme?).
Now during your actual singing you want to avoid being either flat or sharp. Flat is where your voice is lower than the pitch should be, while sharp is the exact opposite. To easily control these think of loosening your body when you go lower while singing, and tighten slightly when you go higher. This is due to the dynamics of the pitches themselves, listen and be able to hear when you are right and when you are wrong. Great habit to have while you are singing.
Now you have to find your natural tone while you sing. This is easy to do just find the most comfortable range for yourself and that is your natural tone. Project your voice to your audience. And be a performer, have fun while you sing, just like the stars.