Rank up your vanguard for vanguard commendations, or rank up a faction (crucible, new monarchy, dead orbit, future war cult) for crucible commendations. Just make sure you are wearing the proper class armour (cloak, mark, bond), and do every bounty you can complete, public events, patrol missions, higher level strikes, crucible ect. You also only need the commendations for helmets and chest pieces. So you can use your marks to get gauntlets and leg armour, and rank them up while you rank up your vanguard/faction. Also helps if you have an exotic helmet or chest piece, then you'll only need the opposite.

Level up the faction/vanguard to get one in the mail, vanguard and crusible armor only in dlc