Magento has various built-in payment methods, one of them is Authorize.net. To enable and configure Authorize.net as your payment processor, you should have API Login ID and Transaction Key. You can get these credentials from your Authorize.net account.
1- Create a test account on Authorize.net (https://developer.authorize.net/testaccount/)
2- On successful signup you will see following info, save them at safe place.
- API Login ID
- Transaction Key
3- Login to your Magento Admin panel and navigate to
- System -> Configuration -> Payment Methods
- Expand Authorize.net
- Insert “API Login ID” and “Transaction Key” into this form and make other settings as shown below
6. Save Configuration and you are DONE!