My car is not working

Hi, I have a 1991 pontiac grand prix se and it died while going for a drive one night. We left it there for the night and came back the next day for it. But no matter what we did we couldnt get it working. I got it towed home and iv been trying everything to get her to work. Now I am not good with cars. I am completly obliviouse to any thing about cars. All I know is that she wont start. She has a ful tank of gas we have charged the battery. Had people try to boost it. So I was thinking it might be the timing belt. Thats what a friend said it could be. So how could I fix this problem and do you know how much it would cost me to get it fixed? Thanks