To join Local Tennis Leagues, you need to create a profile here. This will give you access to an account page where you can manage your Leagues and sign up to a Round.
During this process, you will be prompted to get an LTA Number which you can find more info about here. If you already have this, you can go ahead and create a profile. Getting an LTA Number will also provide you with an ITF World Tennis Number, which helps us understand your playing standard, so we can place you in the correct group.
See here for more info about the ITF World Tennis Number.