I'd set up your PayPal account first - go to www.paypal.com and follow their very straightforward instructions. You need to do that first because to make sure it's you, they deposit a few pence in your bank account - which you have to acknowledge before they will let you do any proper transactions, and I think that can take a couple of days.
Once you've done that, and you want to buy off Amazon (or indeed any site e.g. eBay which takes PayPal) you are all set up, all you need to do is tick the PayPal box on the payment page.
If there isn't enough in your PayPal account for what you want to buy, all they do is debit your bank account instead - but it always shows as PayPal on your statement so you know what it is - and more importantly, you never have to give your bank details to people you're buying from.
Good luck!