The first thing you need to do is visit your authorize.net account settings and get your API user id and password. Here is the link: https://account.authorize.net/ui/themes/anet/User/TransactionKey.aspx
Once you get the api user and pass go to your advanced store settings page - storesetup.wws page and click the 'payment' tab. Choose authorize.net and enter the Authorize.net API user and PASSWORD that you got from the Authorize.net settings page. That should be all there is to it!
ps: If you do not have an authorize.net account you can get one through us. Click here.
