Keep playing the campaign. You'll hit the Dark Below quest line at about level 10, and House of Wolves some time later. By the time you've done them and the other original story missions, you'll be level 25. Or sooner if you run Crucible or patrols, etc.
You know, I know some will claim they're being greedy... But I feel like with the revamped quest system among other things... The reasons for needing to own TDB and HoW start to make sense.