The trouble is that it really depends on how far you wish to go. For instance, the study of bidding simply to bid well is different from that of being prepared for the opponents.
It is safe to say that, for the majority of club and tournament play, having some familiarity with the following conventions will suffice:
Two Way New Minor Forcing
Takeout Doubles (& variations)
Cuebid Limit Raises
Help Suit Game Tries (Interim Bids in general)
Multi-Landy / DONT (or Some Defense to NT Openings
Jacoby 2N, Bergen, & Variations
There's plenty of other high-tech gadgetry out there that is highly beneficial to use and very playable. However, this list takes care of the most important conventions. Notice that all of these conventions either improve game bidding or competitive bidding.