Authorized.net Gateway error: (TESTMODE) The merchant login ID or password is invalid or the account is inactive in magento

Create test account and use
Go do  Magento Dashboard:

1. Go to System -> Configuration
2. Select Sales -> Payment Method (in the left sidebar)
3. Expand the “Authorize.net
4. Change the “Gateway URL” to https://test.authorize.net/gateway/transact.dll
5. Click “Save Config” at the top.