Southwest now boards in two terminals at PHL, I think they're D and E. But you'll need to go to the main terminal for Southwest (it's marked outside) to check the monitors to see where your flight leaves from. You also need to go to the main terminal to check any luggage.
When you get to the gate you'll see entrances for pre-board, A, B, and C. Usually people start lining up about an hour beforehand, but it's really your choice whether you want to wait until closer to boarding time or not. They only issue something like 40 A boarding passes and 40 B boarding passes, so there should be plenty of good seats available for you. And since people are checking more luggage now, there shouldn't be a problem with on-board storage.