I have a 2001 Kia Optima Lx 2.4. The car will not start unless,

Fuel filter maybe

Code came back as lean fuel ? what code P0 what ?

You will need to have the electronics checked for codes and idle control issues by the dealer or a shop that knows Hyundai (the maker of KIA cars) products

Two things come to mind. You broke the pressure seal to the fuel system when you took off the gas cap. The fresh gas rushing in disturbs any dirt or debris on the bottom of the tank.
Next time you use the car when you arrive at our destination, simulate what you do when you fuel up. This will mimic everythig except the new fuel rushing in. If it is still hard to start you know it's something to do with the evaporative fuel system. If not, you need the fuel tank inspected .

The 01 Optima had the keyless entry buttons right on the key itself. I'm not sure if the LX had the option... it was standard on the SE version. In any case, programming is done at the dealership, and is expensive. They use their special "code reader / writer" that plugs into the OBD II port under the dash. You provide all keys and they are recoded to the module that is located under the stereo. A cheaper option might be an aftermarket alarm installed at a local car audio shop. They are less expensive than you might think, even installed. Good luck.
P.S. the 01 Optima remote key had a problem with very short range. You needed to be within a few feet of the doors for it to work.

have the car scanned for the codes,then get back to me,most big chain parts stores will do this for no charge

it would be spark plugs /wires or alternator could be shot which will drain your battery

push button twice

clean the throttle body and the fuel injectors. the problem will be solved.

This could be multiple issues.
1.) timing belt
2.) Crank sensor
3.) Fuel pump (although usually they do not just die they get weak)

Check these let me know.