If your english is average you may just be able to get away with it in first year. However to be successful you will need to be able to accurately summarise cases of varying complexity and present arguments or advise in a manner which is concise.
More than likely your university will have one or several sessions on essay writing and referencing. Example papers are a great way to really get a feel of how your essays should be, at least for first year.
It is crucial that you understand how to reference using OSCOLA and do not leave referencing till the last minute, however the uni may be lenient for the first year. Use a variety of academic sources i.e. books, law journals, cases & judgements etc, in first and second year the professors generally are focusing on your ability to present information from established work and are not too interested in your opinion (journals are v.important for second and third year).
Check with each professor regarding drafts.
Regarding exams, universities have different policies. If you have a legitimate medical condition such as dyslexia then it is more than likely that you will either be allowed more time or some other provision.
Personally, I didn't take my first year seriously but did rather well... I made the mistake of expecting the same results during my second year but unsurprisingly my results reflected the (lack of) quality of my work.
Developing your arguments will take time, but reading journals regularly will help you with this.