How to reach Power 260+
This bit's relatively simple. Whatever your Power level happens to be when you hit Level 20, you can grind your gear level all the way up to around 265 by completing whatever content floats your boat and you're able to complete quickly.
That means farming
Public Events, completing Strikes, diving into the
Crucible for PVP matches and so on. If it rewards an Engram, it's good for the process. We'll stress again though that you should keep hold of any faction tokens you've been picking up until you're Level 265.
Levelling your Power from 265-270
Once you've hit Power 265, it's recommended that you start handing in all of those faction/location tokens you've been hoarding to the various NPCs. If you've been diligently beavering away at events around the world, you may well have enough at this point to hit 270.
How to increase your Power past 270
When you're Power 270, do the Nightfall Strike that's active and open up the Luminous Engram reward to get an easy two or three extra points of Power. You can also get Luminous Engrams from completing the campaign, levelling up with your
Clan and completing the Crucible quest.
Once that's done, do the following four Exotic Quests as these will give you a huge boost:
Once you've finished with the Exotic Quests, start doing Cayde's Treasure Maps. You can do five of these a week, and they reward you with really powerful gear on completion.
Increasing your Power level from 280+
When you reach Power level 280, you can start purchasing
Legendary Mods which should grant an extra five points or so to each piece of equipment you upgrade.