i was in your boat about a month ago, and i've already learned so much. it all depends on what kind of riding you depend on doing. downhill? cruising? carving? dancing? there are different board for each kind, and then there are also more versatile ones for everything. i have a sector9 right now, which is ok, it's a good board to start out on. i got it for $160. look around some of these sites:
www.loadedboards.com
www.longboardlarry.com
www.sector9.com