AIB - Online shopping for the first time

Nope it's not difficult at all. You just need the details on your debit card (the long number on the front, the date of expiry, and the CVV code, which is the last three digits in the number on the back of the card). In some cases, Visa might require an additional check (for example, sending you a text message), but the website you're buying from will lead you through anything it needs you to do.

I don't know if it's still a thing, but previously you needed to use your debit card in an ATM to activate it before you use it online. This is just a once-off thing to make sure no one's intercepted your card in the post (because it proves that whoever has the card also has the PIN).