Hopefully somebody with more experience will reply, but I know if you go to the VirtueMart admin > Store > List Payment Methods > the one called "Credit Card" with code: "AN" is the Authorize.Net payment method module.
You need a merchant account with Authorize.Net, which is set up at
www.authorize.net. I'm not sure how to set up said account, as I've never actually had to do it myself. Once you get the A.N merchant account set up, they will give you all the numbers and info you'll need to set up the AN payment method module in VirtueMart.
Joseph