If it's the first top up, you will need to use a voucher, After that, it will be registered and you can top up via any route in the guide shown above by @Bambino
Info about activating your sim here https://www.o2.co.uk/help/phones-sims-and-devices/activating-your-sim
Best of luck and welcome to the forum
Good morning @Diane6, and welcome to the forum There's a lot of useful information above, which I hope will help you get this sorted. Please do let us know how you get on and if you have any other questions!
→ Access for You: Registration - Find out how to register for our Access for You service.
→ Want to chat with other fellow-minded members? Head to our Off-topic section for some interesting chit-chat.
→ Check out our Priority board for tickets & offers updates, and to discuss all things Priority-related!
→ Welcome to O2! - New to O2? Find out all you need to know to get started!
If you'd like to take part, why not register?
Track usage, upgrade and get support with the My O2 appDownload My O2