Okay, this is kind of hard to do over typing, but I'll try. I'm a music ed student at college, and I'm not a vocalist, but I do know the basics of singing and have managed to increase my range significantly over this past semester.
Make sure you have proper posture, stand up straight making sure your feet are about hip-width apart, aline your knees above your feet (but don't lock them lol) this next part sounds weird, but you have to "open your hips" like, pretend that there are eyes looking forward on your hips, and they're closed when you stand normally, and you have to open them. Shoulders should be aligned over your hips, back straight, and head up nice and high (but don't tilt your head forward or backward) last, hold your sternum up (that bone that holds your ribcage together in the middle), pretend there's a chain holding you up from the ceiling that's attached to your sternum, like it's being lifted.
Range wise, find a piano and start with your low range
On a 5 note scale from sol, fa, mi, re, do (going down from G4) go down chromatically from G4 to F#4 to F4 etc. Starting on the syllable "ooo", when that gets a little hard, switch to "oh" and then to "ah"
To expand upper range, there are a few exercises I can recommend:
1. do, mi, re, fa, me, sol, fa, re, do (1,3,2,4,3,5,4,2,1) Start on middle C (C4) and go up by half steps until you can't go any higher
2. I love, I love, I love to sing (sol, do, sol, do, sol, do, sol, mi, do) (5,8,5,8,5,8,5,3,1) up by half steps from G4
It's very important to do warm ups before practicing your song, make sure you're all nice and relaxed, breathing properly, all that good stuff. This is a warm up that we do in our vocal perspectives class (every thing I mentioned every day) and it has helped me a lot.
I hope this helps some, mostly, expanding range just takes time. If you still can't get it and you're having problems, go talk to your director/conductor, they can probably help you more than I. Also, if you're really serious about continuing singing, get yourself some voice lessons that you go to every week, this will help you a lot with your range, skills and getting into contests etc, in the future :)