I would suggest reading the notes on developers in my FAQ. You've got plenty of good suggestions and explanations here but I have one thing to add: choose ONE developer and learn it inside-out, i.e. see how its behaviour changes with dilution and agitation patterns. Certainly try a couple of developers, but concentrate 90% of your energy on just one and you will get much better results. I would suggest running at least 20 rolls through any given developer to make sure you have a good feel for how it behaves.
The common options:
- D76 is the can't-go-wrong classic
- XTOL is very much like D76 but a tiny bit better in most ways (resolution, grain magnitude, achieved film-speed)
- Rodinal is a non-solvent developer, therefore grainier and sometimes sharper. Extremely flexible for H-D curve manipulations.
They're all good in their own way; you have to find which one suits your artistic goals best and perfect its use. No one really agrees on which is best.
I strongly recommend Way Beyond Monochrome. It'll give you every bit of information you need to make excellent negatives and prints - way better than a bunch of us waffling on the internet.