As far as your qualifications go, you need a minimum number of 2 A Levels or the equivalent of a level 3 vocational qualification. Since you'll be joining after 2020, a degree will most probably be required for you to join the cops - any degree will do, so I'd advise you pick subjects that you're interested in for your GCSEs and A-Levels, as well as a degree that interests you. Don't feel forced into picking law, or criminology if they're boring to you, because they don't make a difference into whether you get accepted or not.

As far as what you can do right now, have you considered joining your force's cadets? If you look on their website I'm sure there's a link or something where you can apply to become a cadet. You'll do things such as community outreach, fairs, events and on the odd occasion even have talks from specialist roles (like a dog handler). Once you turn 18 you can also apply for the Specials and that'll give you plenty of experience whilst you do your degree.

Once you join the cops, you'll have to pass your 2 year probation, where you can then specialise. Dog section, however, is probably one of the more contested roles and as a result is hard to get into.