To reach max power level in Destiny 2, you will need to get gear with higher power levels. Items with a higher power value will help raise your overall power level.
But equipping just one item won't be enough, as one high-level item will just raise your level by a small amount. Because of this, you will need to fully equip yourself with high-level gear, which means finding better items for all of your weapons and equipment slots.
That said, the best way to get gear with higher power values is to play through all of the campaigns in Destiny 2. Farming the campaigns will allow you to regularly gain new items to increase your power level.
These new items will also increase in level as you raise your power level in Destiny 2. Because of this, it's best to farm all of the quests and equip both new armor and weapons if they have a higher level than what you currently have.
You also will get opportunities to gain legendries and even a few exotics from farming the campaigns in the game. Both can have decent power values when they first drop. But as you progress, you may find these items becoming obsolete due to their low power values.
In these cases, we recommend saving them for later as you can raise their level through upgrading. Some weapons and armor can also have decent stats, so that is another reason to keep these items for later.
For more Destiny 2 guides, check out How to stack bounties in preparation for The Witch Queen in Destiny 2 right here on Pro Game Guides.