A lot depends on what sounds you're looking for. To start with, I would suggest a chorus pedal (Boss CH-1 for example), a delay pedal (boss DD-3 for example), and maybe a wah pedal. A wah tends to be more useful if you play lead guitar, but I play rythm and find plenty of uses for it. You also might want to look into an overdrive or distortion pedal if you need more dirty tones than what your amp can provide. A good (from what I hear) overdrive/distortion pedal is the Boss OD-20. It is a multi-effects pedal, in that it models several different types of distortion pedals. Also, once you get into using pedals, don't skimp on the cables. Get some decent quality cables. Lower quality cables will work, but they will deaden the sound a bit. I like George L, Planet Waves, and Livewire cables.