Could be a number of things - from a bad fuel pump and/or pressure regulator, clogged fuel filter, faulty injector(s), clogged air filter, faulty induction system, vacuum leak, worn spark plugs, electrical issue, problem with the ECM, dirty throttle body, bad gas, poor engine compression, valve timing issue, etc.
When you say hard to start - how long do you crank the engine before it starts? Does it crank well (strong, fast cranking) or does it crank poorly (slow, labored cranking)? When did you first notice this issue, any recent repairs/work done on the car that seemed to coincide with that time?