At the payment service’s details page you setup your payment method settings and enter the credentials for the selected payment method.
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 settings section:

At the 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.
Please note: 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 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.
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 vendor selects this payment method when paying for vendor services.
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 vendor selects this payment method when paying for a service.
e.g. a simple thank you message.
After you setup the service payment method, press the save button at the bottom of the page to save the changes.