Do the Story Missions
Before you start going crazy over increasing your light level to hit the new level cap in Destiny: Rise of Iron, first things first: beat the story missions. The campaign in Rise of Iron only requires you to be at level 40 (and there’s a level boost item for those just jumping into the fray now), so you should be able to start the story missions immediately upon launching the game.
You’ll be able to enjoy the new story content that Destiny has to offer, and you’ll certainly pick up lots of new gear and weapons to help increase your light level as well. Before you even think about grinding the strikes and tackling the Nightfall, make sure you knock out the story missions first to get your hands on some of the new weapons Rise of Iron has to offer.