Well, ideally you should have some friends/aquaintances who also play. You form a team, make your own player (in 18 there was a new ‘quick’ feature where you could make three versions of your player - three positions, i.e St, SAM, CM)
You control your player, and your friends control theirs. You can choose to have one of you control ‘the rest’, or let the AI do that. You have to be two players minimum (which is too few and boring ofc).
You rank your player up by points and experience. Points for special boosts, xp for playing games (scoring, passing, tackling, crossing etc). Your team follows a league system 10. - premier division, like in single season mode.
Edit: The points give you special advantages, like ‘flair’, ‘second wind’ etc. You always start with a given amount to get going (rank up from 73tot to around 77tot)
Have played it since it was introduced. Used to be manic about winning single mode seasons in the past, but on 17&18 I’ve only played single to get acquainted with the gameplay.
Puh! Sorry for the long reply (less is more I know
). Generally it’s great fun while you chat shit with your mates.