I presume your 17/18 now??
Start going through the process now, by the time you've had your initial chat and started the process its going to take 4-5 months minimum before ADSC. All tests last a year, so does medical so fits in perfect, even if you pass at ADS you can set an Earliest Enlistment date for when your A levels finish.
Have a look in the stickies, especially recruiters tips ones then nip into the ACIO and have a chat...worth taking your parents in aswell if thye have any questions.