My mountain bike wont shift into second or third gear?

I am stuck in first gear, i can go through all the first gear variants but i can't shift into second or third gear.. i just bought the bike to day and stupid me i didn't go through all the gears before purchasing. the bike shop is on the other end of town so i want to try to fix this myself. it's a GT avalanche 3.0 if that matters. Any help would be much appreciated