The jobs you get to choose from depend on how fit you are when you apply - physically and mentally.
IMPORTANT: If you're not very fit when you join up then you run a high risk of being injured in training and having to leave. This is because unfit people change the way they run, which puts extra stress the body, particularly the legs.
The risk is highest if you're aged 16 or 17, because your bones are still growing and will struggle to cope with running long distances with a pack on your back, especially if you're not very fit now. This is another reason to wait until you're older before signing up.
The fitter you are, the more jobs you get to choose from in the army.
If you're aged between 16 and 17.5 when you join up, then to get through selection you'll have to be able to run a mile and a half in 14min, 30sec. If you're older, then you'll need to run the same distance in 14min, 0sec. Run it faster and you get more jobs to choose from.
You'll need to lift a 20kg bag with two hands and put it on a shelf at head height. More jobs are open to you if you can lift a 40kg bag.
And you'll need to carry two full 20-litre jerry cans for 30 metres without putting them down. Again, if you carry them further you'll get to choose from more jobs.
When you're older, you're stronger, which is (yet) another good reason to wait until you're 18 or older before signing up.
Your mental fitness also counts. If you have good GCSEs and do well in the army's tests, then more army jobs are open to you. Otherwise, you get offered the least popular jobs.
You can practise the army's basic 'BARB test' online.
If you want to join a technical army trade (e.g. signals, mechanic, electrician...) then you'll be given harder tests. You can practise the technical test online.
So, again, if you stay in school or college at 16 and then sign up when you're 18 or older, you have a better chance of doing well in all these tests. Then you'll get to choose from a wider range of army jobs, as well as jobs in the navy and RAF.
You also have to pass a medical. If you have certain medical problems, or mental health problems, then you won't be allowed to join. It's a long list but you can read on pages 6 and 7 of the army's guidance notes, online.
If you decide you don't like the army, do you know when you could leave? Find out about the rules for leaving the army next.