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