What is wrong with my car? Spurting engine in low gear and small pop sound?

I have a 99 Chevy prism 4 cyl. I would just like to have an idea whats wrong with my car before i take it into a garage and so i don't get ripped off. When i start the car and get it going from a stop the engine sputters or skips till i get up to about 40mph then it's a steady. Around 10 to 20 MPH the engine does a small popping sound it sounds like I hit a paper cup with my axle. The engine light blinks when i'm going up a hill like it's not getting enough power to go. Any ideas?