** HOW TO INSTALL MODS **
https://forums.bistudio.com/topic/141590-beginners-guidehow-do-i-install-modsaddons/
Little guide for those who do not know how to install manually (not mine, just a resource).

Most if not all mods published for Arma 3 will have a folder with all the addons inside already so it comes down to simply renaming the folder and placing it where ever you have your mods at (some may require more so read the instructions on the mod page to double check).
Next you open the Steam Arma 3 Launcher (launch Arma 3 from Steam) and select the mods tab on the left and then click on the "Local Mod" tab at the top. Direct this to whichever folder you have your Arma 3 mods located (it may take a little bit depending on how many you have to load up). Then check them and start the game.

** WHERE TO GET MODS **
http://www.armaholic.com/index.php - Biggest resource that I know of for Arma mods.
https://withsix.com/p/Arma-3 - Should have most if not all the mods on the former and others which may not be published on Armaholic. It has it's own downloading software that you can either get in the form of a launcher or website plugin. Download speed is decent.

Either way you want to direct the Steam Arma 3 Launcher to the folder where the mods are located and activate them before launching.

** P.S. **
You need to direct the Local Mod tab in the Steam Launcher to your mod folder everytime you get a new mod so it can dectect them and you can activate them.