For musicians are they trained on the job or do they go to music school first?

As part of your initial trade training/ phase 2 training, you will attend the Alford schools of Military music in Portsmouth. Whilst there you will be taught everything necessary for the job and complete your first Career Employment Qualification, you will then complete a further two exams whilst in a band.

So I can join with minimum musical experience? Eg. I play the guitar and I consider myself as an advanced beginner.

You would need to pass an audition as part of the application process, your suitability would be decided then. The audition includes playing a couple of varying pieces, a short theory exam and some aural perception.