Welcome! I do the opposite commute daily, typically using a monthly pass, but I've also purchased ten-ride passes in months where I'm not going to be at the office very much. Either pass works the same - you get a paper ticket (sadly no etickets yet for the passes, so you need to keep track of that very valuable piece of paper), and then you hop on any NE Regional or Keystone between PHL and NYP, with the exception of a few restricted trains (and, of course, no Acelas). No need to book anything ahead of time, just hop on the train and go. With the ten-ride, the conductor will punch a hole in your ticket for each of the ten rides, too.

If your commute is predictable, you can in fact save some money by using advance-purchase tickets, but you'll get stuck paying the walk-up fare if you have to switch, so I prefer the passes for the flexibility.

Let me know if you have any other questions, because I've literally made this trip hundreds of times now.