You can ask if the customer has their own invoice form that they would like you to use, especially if you were working for a company. if not, it's fine to adapt a form that you can find free online. There is an example here that gives you full instructions on what to include, as well as showing you a sample that you can use, but if you put 'uk invoice template' in a search engine you will find others too.

The main items to include are all prices, any VAT or extra cost and the payment details. You need to show how the invoice can be paid - ie, give your bank account details, paypal address or whatever applies in your case. You should also give each invoice a number, and you need a system for keeping track of your invoice numbers. There are various solutions for this or you can devise your own system.