First of all, don't panic. They are not going to expect full marks from you since most applicants don't have much experience with law in school or before uni. However, some prep would be useful. I did the LNAT last year and I remember there was something you could download to practise questions in exam conditions - I think you can download it from the LNAT website. This helped me to gain an experience of what the test would be like and how to organise the time given to you. Other than that, maybe read a couple of newspapers and explore current affairs or events as this may be relevant in the essay/written question (I remember mine being something to do with privacy laws on social media).
I think that's all the advice I can give you at this stage. Good luck and don't panic