Strange sick smell coming from inside the car?

i just recently purchased a 95 ford escort. I dont know a whole lot about cars, just begun slowly learning. When im driving around, theres this smell of gas and monoxide inside the car that makes me a bit dizzy and sick so i roll down the windows. I noticed its even worse if i use the heater/ air conditioning... my question, What could be causing this problem? is it fixable? can i fix it myself? how much money are we looking at in repairs? thanks.