You do not need to study law during your A-Levels, or the equivalent for 16-18 year olds in other countries, but core subjects such as English will help your communication skills. It is a good idea to take a broad range of subjects to keep your options open. If Law is available to you, then you should take it as it will give you a clear understanding of the subject and confirm whether or not you wish to study it in higher education.
You will then need a law degree, which could take 3, 4 or even 5 years depending where you wish to study. If you have any colleges or universities in mind already then you should check their website to see if there are subjects they require you have previously studied. Good grades across a broad curriculum will generally be what you need to study law at university.
Work experience is also vital to begin a career in law. Begin your work experience as early as possible and do your best to find a placement during every vacation, as this will definitely help you in the future. It may be difficult to find your first placement so start at a small firm and once you have a few placements completed, you will have more chance of being accepted into a bigger firm.