Good sources of XP:
- Daily and weekly PvP, GSF, On-rails Space missions.
- Daily and weekly Group Finder missions (Flashpoints and Tacticals).
- Daily and weekly Flashpoint missions (combine with the above).
Notes on the XP boosts: You can save up to 3 mission rewards in your pending reward queue before using the boost, and also wait to turn in completed missions at terminals (for low levels that includes daily/weekly PvP and Flashpoint stuff, not sure about planetary stuff). This works well with the generic boosts. The more specific boosts (Flashpoint, PvP, GSF, Space missions, story missions) are quite cheap on the GTN because they come in abundance from cartel packs, so those are always worth using whenever you do an hour of any of those activities and/or if you can't afford the generic ones. Boosts don't stack though.