It varies. Mountains of reading where the lectures will provide a skeleton and you have to flesh it out by yourself. You may be given an essay to hand in every week or so for tutorials/supervisions. You may get an assignment for a module which counts for a certain percentage of your final grade. Most, if not all, universities will have exams that count for your entire grade or part of it.
Mooting is essentially a society. Some universities have an assessment for mooting which is mandatory but after that, it's up to you if you want to take part in mooting competitions.
Do law if you like it or are passionate about it. Many people drop out of Law because they were unsure about the subject and what it entailed going into it.