Definitely hook up with a good driver if you are going to Ted's or Orange Hill (particularly Orange Hill since it is not within walking distance of Negril). You might find the local scene a little intimidating on your first trip and a good driver will stick with you and help you navigate the craziness. Shane Taxi is a good choice. I would also recommend Michael's On-Time Taxi - his info can be found in the Transport link at the top of this page.

Not much of a traditional Spring Break scene in Negril, but it sounds like it has the vibe you're looking for. You will probably find some people your age at Margaritaville, but as Flip said don't spend too much time there as there are 7 miles of beach (plus the cliffs) to explore.