There is information booth somewhere near Sheraton and Carnival Pier in Old San Juan. Staff there provide any kind of touristic info you want. Right next to it there is also a bus stop of "historic bus" (free) which will take you to the major tourist attractions like both fortresses and so on.. However if you don't want to prepaln a lot, audio tour may be a good choice if it's not overpriced of course)
I was in Puerto Rico twice: both in San Juan. I walked there on my own.
If you area good driver, I suggest to rent a car: even a one day trip to the farthest south or east of the island will be worth doing