Engine lean=hard starting up?

Well I have a hard time to get the car to start anytime, and I was wondering if this problem could be caused by it being to lean.  Why would the car be lean anyway if I replaced the O2 censor and all the plugs-n-wires?  Fuel injectors are clean, plus a new fuel filter and pump.  I think it could be a leak or the EGR valve is bad but I can't tell.  So I need to know if their is another sensor is bad or what the hell is going to happen if I don't fix the P.O.S.

what year/model is the car and do you have any check engine light. Also how do you know its lean anyway

the infor that TheEngineer is what i need to know as well but A solution is an SAFCII and just add a lil fuel.