The surest way is to level up reputation with the vanguard or crucible, or if you buy and equip a bond from one of the factions (New Monarchy, Dead Orbit and Future War Cult) and level them up to rank two you can buy gear with marks. Just get your bounties done, play the strike playlists, etc. The game doesn't exactly throw legendaries at you like others. I would suggest investing in the chest that Xur is selling this weekend if you can spare the coins. A pretty good roll on the strength stat and it is a nifty exotic for the warlock. Oh, and even if you don't get anything until you buy a piece, all blue gear can get you up to level 24 for the most difficult strike playlist.

Hike up your shorts and grind bounties. I'd advise you getting a faction like Dead Orbit or something because everything you do will get you rep Or you can get Vanguard rep because the gear is super easy to get for that In either case, get a set of level 20 blue gear, max it to 24 and then do the Tiger strikes. Enough of them and you'll be good. Might even get legendary engrams.