The first thing to note is that everyone playing Destiny can now reach level 31, whether or not you buy The Dark Below. The key is the new legendary (purple) armour being sold around the Tower. Bungie has gotten rid of the old stock held by the Vanguard, Crucible and Faction vendors and replaced it with new legendary items that have a higher potential light level. The old gear could only deliver 27 light. The new stuff goes to 33. In order to hit level 31, you need a total of 132 light, of course, you can also level up fast through destiny power leveling. Conveniently enough, four pieces of the new armour will give you that, because 33×4=132. (Who needs Brain Training, eh?)
If you already have a lot of Vanguard and Crucible Marks and have ranked up the vendors sufficiently, you can probably buy a few things immediately. Vanguard, Crucible and Faction vendors (New Monarchy, Future War Cult and Dead Orbit) all sell new gauntlets, chest and leg armour for 75 Marks apiece if you are rank 2, while helmets are 120 Marks and require rank 3 and a Commendation. We’ll come to Commendations in a second.
If you’re a little short on Marks, then you will need to grind them before you can buy things. One good way to obtain Vanguard Marks is to chase Public Events using destinypublicevents.com; successfully completed Public Events reward you with four Vanguard Marks and you get another two (plus an ascendant material) for your first event of the day. The other way is to take part in Vanguard Strike playlists, which reward Marks in a steady stream. It is also worth completing the new Dark Below story content so that you can unlock the Will of Crota Strike, which offers up 20 Vanguard Marks the first time you finish it.
For Crucible Marks, you just need to play a lot of Crucible. Remember to take on the daily playlist promoted on the navigation screen for a slight boost.
When you have four pieces of the new legendary armour, all you need to do is level them up to the maximum. It will take time to obtain the necessary amount of XP, not to mention the planetary and ascendant materials required to finish them off, but if you’ve gotten this far then you are undoubtedly used to the Destiny grind by now. Welcome to more of it! Don’t forget to finish the Nightfall to gain that weeklong XP boost.