I don't know if you have chosen A levels yet but things like English Lit and History are good ones. I did those two plus German A levels. In other words traditional fairly hard A levels, even maths and sciences are fine, are best as law is quite academically competitive.
50% of future lawyers do not do law as a first degree by the way so do bear that in mind too although I very much enjoyed my LLB.
You will need to do the LNAT exam if you choose law for most decent universities so practice at that is a good idea.