Do story missions. If they become too hard, patrol (green arrow on destination map) for an extra level or two, and retry. Do all bounties you can. Pvp if you enjoy it; it's solid exp. When you hit 18 and on, run highest strike playlist you can manage, and save up marks for gear once you reach the required level (which you will get from strikes and bounties for Vanguard rank and from Crucible for Crucible rank). Once you hit 20, run strikes to get blue gear with highest +light level. Buy purple armor as soon as you can for even higher +light.