The basic post 20 strategy, is to get marks. Do Strike playlist, play crucible, do public events (destiny public event tracker if you're looking to do this), and use those marks to buy legendary gear from the Vanguard.
Make sure you do every bounty you can (it's pretty easy to do 5 or 6 in an hour) to get your rank up.
When you hit the plateau levels, ask some of the guys on here to help you out to get you through higher level content, to get you either strange coins to buy your first exotic, or pull you through the Raid.
For all blue armour, light 15 on each, you will be able to hit 24 to do the strike playlist at the highest level, so you can get your first legendary piece. After that, with 3 blue pieces, and a legendary, you can hit 25, and be pulled through the raid by some rcfers, or friends.
Then you pray to RNGesus, get some Raid gear, get up to the high 20's, start getting your exotic (unless you got lucky and already nabbed an exotic).
For example, casually playing a Warlock, I got it from creation to level 28 within two weeks, only buying legendaries, and doing the raid. The main thing is having people who are willing to help you out with stuff that is above your level. If you don't, because of the mark caps, you will hit some roadblocks.
Good luck.