It's always good to take a head start if you want but i'd say when you begin AS/A-levels revise from the beginning of the term but but chill and take it slow at first, when it comes to serious revising start doing that in January at latest, you won't be able to remember everything you need to know for the exam if you leave it til april/may
But right now,, make the most of your summer young one,,, cherish it... pls
my tips:
-Download every past paper or specimen paper and the mark schemes, and keep them sorted in a folder for you to start doing after you've collected/written all your revision notes! (for most subjects, the old spec and new spec are like 70% the same, unless its a subject like psychology, i took a third year for biology and chemistry and saw what my class who are doing the new alevels being taught and alot of the content is the same, even using the same powerpoint files my teacher taught us with the year before, so even downloading from the specification that isnt yours will be useful)
-textbooks have way too much information and you only want the relevant stuff that will be marking points in an exam, i'd say go through the textbook slowly as the months progess and buy exercise books for each exam unit from ya local WHSmith or Ryans, and write all you need for the exam into them! This is specifically for subjects which require a lot of recalling, less for subjects like english or psychology involving lots of essay style stuff
teachers will say it to you a lot but past papers are the only way to familiarize with yourself with the way the exam board questions you, and you'll notice what sorts of information from the textbooks are important from them too :3