All you need to start playing in competitions is to be a British Tennis Member, this gives you a membership number and player rating, which helps identify you and makes sure your match results are recorded.
Find out more about British Tennis Membership
It is possible to enter and pay for most competitions online and parents and coaches can do this on a player's behalf. Where online entry is not available you will need to complete an entry form. When you find a competition using the competition search there should be information about the entry process and requirements.