Just join a server and practise. The multiplayer is not like single player. It's always battles (no campain screen,Towns,Castles etc)
there are different game modes such as:
Deathmatch- Free for all where the person who kills the most wins
Team deathmatch- Like above but with two teams,the team that kills most wins
Battle- Like team deathmatch apart from with no respawns.team that kills all of the opposing force wins.
Siege-Two teams,one attacking one defending. Defenders need to hold the castle until time runs out or enemy has no respawns left. Attackers try to capture the castle in the round time.
there are other game modes just look arround and find the one you enjoy the most. Have fun
