I often like to fly shorter routes, KLGA-KPHL for example, and I recently started flying a VATSIM. Sometimes though, my route doesn't contain which SIDS/STARS to fly. What should I do when this happens?

Flying in the world of simulations, I don't think it'll matter very much. However, in real life, you shouldn't worry. A SID, or Standard Instrument Departure, is just to keep traffic flowing, they're often found at busier airports. If one hasn't been included in your IFR clearance, than either one doesn't exist for that airport, or the controller wants you to follow a different departure that they will outline thoroughly in your clearance. As for STARs, again, similar thing. These are just Standard Arrivals, and so they help get airplanes to the initial fix, so they can begin an approach. If one hasn't been issued, it may be simply because one doesn't exist for that airport, or the controller wants you to follow a different procedure that they'll outline thoroughly for you, haha (that almost seems like Deja Vu). You can always request a SID or a STAR, as well as an approach. If the controller is vaguely clearing you for a STAR or SID at an airport with multiple STARs or SIDs, then you can always ask for clarification. You can also file SIDs and STARs in your flight plan. Another trick to get to the fix of an approach is to simply request for Vectors, which works great when you have an emergency of some sort. A controller can always deny your request, but there is no harm in asking.