Chris, concentrate on your school work and do the very best you can. Then do some very solid research into the armed forces and roles using the official careers sites. They are a lot better than they were a few years ago, and provide pretty much all the information you could want to make a decision.
However, once you've narrowed down to a particular role, do come back and ask focussed questions which the experienced people on here can answer.
And in reply to another thread you started, can I stick my oar in and recommend that spelling, grammar and punctuation are really important in life (not just exams) as a pillar of communication. If you communicate badly, things do not go well, and you rarely achieve what you want in life.
Just look around you, and see what lazy talking and writing skills do for people. Just because a lot of people talk.or write in slang with no apparent rules, doesn't make it right. How people talk to you or email you in professional life (ie work) tells you an awful lot about them and how much credibility they have. If you want to join the military you should aim high and be the best you can be 😀
Good luck for your GCSEs
Ikky