To accept online payments, you need to go to Options->General settings->Accept online payments

Then you can set up, by only giving your name

Turn on or of the Use automatically checkbox if you want to use it always or not. You can change this anytime when you send out your invoice. 
If you want to create a account, you need to have a PayPal account, then it only takes a minute. Here you can create one, or click the i button

You can turn on or off here before you send the invoice

This is what your customers will see, at the bottom of the invoice, they can choose PayPal payment, so you get your money faster. Easy and simple, as everything should be!