At the payment method’s details page you setup your payment method settings and enter the credentials for the payment methods.
Though a lot of settings are common throughout a lot of payment gateways and payment methods, the exact names for some fields vary. Please refer to your payment gateway’s documentation for instructions when setting up a payment method.
The common settings section:

At the common settings section you can edit the payment method’s name. This will affect the name of the payment method at the storefront. You can also set the protocol mode for the payment method and change the sorting priority.
It is strongly recommended to use the HTTPS protocol for credit card and check payments. You should use it, if it’s available for your site. Regardless of your setting, some payment gateways will always use HTTPS for their communication, if it HTTPS is mandatory.
The payment server settings section:

The payment server settings section will differ widely, depending on the selected payment method you are editing. Typically, some settings are common among most payment gateways.
Usually you will have an URL to the payment gateway or payment processor. Though a URL is preset for most supported payment methods, you might need to alter that URL, depending on your settings for that payment gateway.
A lot of payment gateways also offer a test or sandbox mode where you can test your settings in a safe environment, before going live with your cart.
You will also need to enter some kind of login or id with which a payment gateway identifies your account and usually some form of authentication like a password or a authentication key.
You might also need to set some sort of verification code, if the payment gateway needs this to verify, that the communication between the shop and the payment server had not been tampered with.
Some payment gateways support multiple payment types through their gateways like “capture” transactions where a customer’s account or credit card is verified to be in a good standing and funds are marked for later settlement or “auth” transactions where a reserve is placed on a customer’s account until an administrator marks the funds as ready for settlement, e.g. after goods have been shipped.
Most payment gateways also offer a setting to select the currency for the payment gateway.
Please note: The shop will try to convert the amount the customer has to pay to the payment methods currency at checkout. Please make sure, that the correct exchange rates for all used currencies have been set up at the currencies page.
The accepted credit cards section:

At the accepted credit cards section, you can setup credit cards for distinct payment gateways. You can set a shortcode, that is used by the payment gateway itself to identify the credit card type, the name of the card that will appear at some select boxes, if you want to use CVV2 and if you want to enable this credit card.
After entering the credit card details press the “Add” button to add the credit to the list of supported credit cards. To remove a credit card from the list of supported cards, click on remove credit card on the right side of each credit card entry.
Press the “Save Changes” button to save any changes on the list.
Please note: This section will only appear, if the payment gateway supports a separate credit card setup.
Please refer to the payment gateways manual when entering the credit card settings, especially the card code or if CVV2 is available.
The payment form title section:

At the payment form title section you can edit the text, that will appear on the payment form , if a customer selects this payment method to pay.
The payment page message section:

At the payment page message section, you can enter a message that will appear at the payment page, if a customer selects this payment method during checkout. e.g. additional information about this payment gateway
The thank you page message section:

At the thank you page message section, you can enter a message that will appear at the thank you for your payment page, if a customer selects this payment method during checkout. e.g. additional information about how an order is handled or how many days an order typically needs to be processed or a simple thank you message.
After you entered all the payment gateway’s information and made the desired changes, press the “Save Changes” button at the bottom of the page to save your settings.