When someone wants to pay you, you have them send the money via paypal to the e-mail address that you registered.
Me: "I'd like a $5 badge please! Here's my character info (blahblahblah), what's your Paypal address"
You: "Sure thing! My paypal is email@example.com
. I'll get started on it as soon as I receive payment."
Me: "Ok. I'll go do that now"
Then your commissioner goes to the Paypal site, clicks the "Send money" link, fills out the form and sends the money.
Me: "All sent, from firstname.lastname@example.org
You: "Yup, I got a receipt from paypal that you sent it. I'll get to work!"
You: "Here's your awesome art, thanks for commissioning me!"
As for taking the money off of your Paypal and into a bank account, you need to check out Paypal's site and get help to set it up.