Completing these will set you up with the basic mechanics of ARMA 3 and the range of keybinds you need to use. If you’re new to ARMA, make sure you complete all of the tutorials, the bootcamp campaign, showcase missions, firing drills, time trials and singleplayer campaign before considering multiplayer.
Because of the tactics involved, it’s essential that you have a solid ground in playing single player before attempting multiplayer.