Mod? For what? If you're talking about Altis Life, it isn't a mod. It's an ARMA3 mission script. All you need is an up-to-date copy of ARMA3, and your fingers (so that you can type 'altislife.co.uk' into the FILTER box on the Multiplayer game browser in ARMA3).
Then just join the session. Your game will automatically download the compacted (well, bundled) mission PBO file, and start the mission. You just choose a civilian 'slot' to connect in, from the menu, and then wait while the game spawns you into the world of Altis.
Oh... and I would heartily recommend reading the rules - both on this forum, and within the game itself (on the M button map-screen, top-left, in the various menus that drop down). Unfortunately, many people dive right in, get a gun and go hunting other civilians or generally shooting the place up, and are disappointed and confused when they get banned. Please don't be a statistic!
Once you're in, feel free to ask around the forums (or here, or even in-game on the text Side-Chat (blue channel) for more help - they're a good bunch of players here, and most of them will happily help you out or offer to let you join their gang (usually after robbing you or subjecting you to some kind of initiation humiliation, granted, but still... it's better to be part of a group than go it alone, often. It's a cruel world out there!)