Does the car start then sputter out immediately after starting?
Do you have any check engine lights?
When you turn your key on to start car, don't turn it all the way to engage the starter. Listen and you should hear a whining sound. This is your fuel pump charging the fuel system. It should run for only a few seconds. If it runs for more than 6 or 8 seconds or if it does not stop then you could have a fuel pump or fuel pressure regulator problem. If it does not come on at all you may have a fuel pump or fuel pump electrical problem
Other things that could cause this is a weak spark from coil pack, worn spark plugs, clogged air filter or vacuum leak.
You can check the simple stuff first. Air filter. Vacuum lines you will have to take the plastic engine cover off for this.
Pull your fuel pump relay or fuse then pull off one spark plug wire and turn over engine hold the end of wire close to something metal (spark plug) you should have a nice blue spark jumping out the end of wire.
If all that stuff is good, pull a couple of plugs out and look at them. If the car has high miles and never had a tune up the plugs may be worn down.
If plugs are good and the other things I mentioned you have other problems and need to have a mechanic look at it.
If you are getting really crappy gas mileage I would concentrate on vacuum leaks and bad plugs or coil packs.