You don't really need anything extensive unless you're really looking to min-max.
Infusing is the option, when you inspect the gun, that lets you basically "eat" another weapon to raise the one you want to a higher Attack. The gun that you consume is typically called "infusion material" or "infusion fuel." The infusion material needs to be the same kind of item (I.e., pulse rifle, shotgun, glove) to infuse. It costs 4 legendary shards to do this (except infusing an exotic into another exotic, which costs no shards).
Should you be doing this? Only on gear you WANT. If there's a gun or armor piece you absolutely love and want to keep, you can infuse it to keep it relevant (all the way until the last moment Destiny 2 exists in this world!). It's not worth doing on "just anything," though.
Your Light level (or "Power" as currently depicted in game) is an average of the attack/defense values on all your gear. At the point in the game you are, you're going to keep getting blue (Rare) gear drops that are higher light than you have equipped.
You can continue to raise your Power level by simply doing activities post 20 that award loot. If you get something that is higher, equip it and watch your numbers go up!
In order to be ready for the raid, it's looking like you want to be at at least 250, and ideally 260.