I know you use SID's when depating and STAR's when arriving...but suppose I am going to depart Laguardia....or any airport(preferablly Unites States)...and using runway 4...well how do I know which SID to use???...I have the departure plate and everything but how do I know which one to use(do I just pick one)...also how do I know what STAR to pick on the arrival I have all the plates(U.S. ones)???....if you can help thanks...

In real life, before departure the ATC will tell you which SID to use, clearance altitude, QNH, squawkcode etc.
Flightsim does all of the above, except handing you a SID.

But there's a trick do figure this out by yourself
Every runway has a number of SIDs associated with them, so by selecting the runway first (assuming you fly with FMS, which I think you do), you automaticly exclude all the SIDS that are not valid for runway 04, for example. Now, if you're lucky your flightplan's first fix might also be a transition from a SID, if not, simply use the "Plan view" (if your aircraft has that option) and tinker with the waypoints manually

Regarding the STARs, you say you have the charts, well just look up the "STAR chart" and you can easily see which STAR(s) to use when approaching from the north etc. Same can be said about the SIDs...In real life ATC will of course tell you which STAR you should use, but then again Flightsim ATC is far from realistic