It will be fairly easy.
First your medical record will be looked at and if satisfactory a simple pre-selection test will happen.
1) As many sit ups in a minute (around 25-30 is passable)
2) As many press ups in a minute (again 25-30 is passable)
3) Grip strenght (done on a kg pressure grip, if you are not a 7 stone weed, you will pass)
4) Standing long jump (feet together and spring into a leap, 3/4 of your body height will be passable)
5) Running test. Either 1 and a half miles in 12 minutes or 7 minutes on the bleep test (two cones in the gym hall running between them with an increasing audio bleep).
That's it.
As long as your bodyweight and height is not too far out and you don't smoke 40 a day, it is not tough.
Just work a little on CV work as the bleep test will test the REALLY unfit ones.