You are in a great spot. There are over thirty yacht clubs on the Delaware River who are members of the Delaware River Yacht League (DRyL). Some are mostly power boats, some are more sailboats. The best way to crew is to contact the clubs and let them know you are intrested. Most clubs have a racing night during the season where people are looking for crew to help out.
It is a great way to learn....and cheap.
Curtin Marina, Burlington, NJ is home to a yacht club that races on Wednesday (I think). Contact them and ask about which night the club races...AND THAT YOU WANT TO CREW.
You can get there by train using PATCO and then taking the river line to the second stop in Burlington, NJ.