This guide will explain the practice behind increasing your power level in Destiny 2. If you do not know what power level is, I would suggest you check out the guide that explains how levels and XP work in Destiny 2.
Power level is a variable level that can go up and down based on what you have gear you have equipped. If you find a better helmet and equip it, your power level will increase. It is as simple as that. There is only one way to increase your power level in Destiny 2 and this is to find better gear. This is of course easier said than done, but the rarer the item, the more of an increase it will provide for your power level.
The calculation for your power level is not based on the amount of defense or armor that each piece provides, it is based on the rarity and other enchantments along with the attack power and offensive increases that come from the other gear that you have equipped on your character.
The best way to increase your power level in Destiny 2 is to go hunting for exotic and legendary gear. If you have the means, you can just buy the gear with real money or in game money and be done with it. This will help you boost your power level considerably. If you do not have the money, you will need to go and do all of the missions you can. Completing every mission on a planet will give you a special quest to obtain some exotic gear and in the process of doing this, you will likely obtain some legendary or exotic loot while killing enemies and looting random chests.
The main thing you need to know about increasing your power level is that you cannot spoof the metric or grind it. Just because you have hit power level 100, does not mean that you are there forever. If you suddenly equip a bunch of garbage loot, you will fall right back down to having level 1.