Light Level
Right now the current Light Level cap is 335, and you’ll want to be as close to that as possible. The best approach to accomplish this is to start running through endgame content as well as PvP events which include the Iron Banner and Trials of Osiris. This will grant you rewards with increasing light as you continue to run through them.
Prison of Elders is one that can be done relatively easy with a group of friends, and the raid can get you several pieces of powerful gear, though it requires six players rather than just three. Just keep running through different challenges to amass a collection of gear you can either swap to or infuse into your favorites. With just over two weeks until the release of Rise of Iron, you should have plenty of time to power up.