That depends on how long your hair is when you join. The Army has a number of requirements regarding hair length.

For women:

Short hair should be 15mm above the collar if you have a bob-style haircut

Long hair should be tied up so that it sits 15mm above the collar

Ponytails can only be worn during personal time in the barracks and when not in uniform

No hair should show on the forehead when a beret or hat is worn.

For men:

Hair should be no longer than 15mm above the collar, and no shorter than a number 2 comb haircut

Sideburns must be neatly-trimmed and not extend below the middle of the ear

Your face should be clean-shaven, although moustaches are allowed provided they do not fall below the top of the upper lip.