Do bounties, and campaign on hardest difficulty you can handle. Do Crucible when you're to low of a level to do the hard campaign. Getting to level 20 is easy if you follow this method.

Play with someone who's higher level and have them rush you through the harder difficulty missions for extra xp. Complete bounties as you do this and then switch over to crucible once you've finished. Hitting level 20 takes roughly 4-6 hours to do legit.

real smart logic there.... when 85% of the time, within 1st 5x replies, thats where you'll get told to go......

Missions/Strikes/Bounties