PayJunction is an online product that one can subscribe to in order to process credit cards (Visa, Mastercard, American Express, Discover.)
You can get your own Merchant Account by filling up our online form.
Once you have an account, you want the PHP PayJunction library and if you have Drupal 6.x, also get the MO PayJunction module. You can also purchase both at once and save 10% with the bundle.
Once you have your PayJunction merchant account, you can start charging your customers for your products and services. In order to get your Internet gateway to work, you want to have such an entry in your PayJunction account. By default, that account may be turned off (it depends on your first setup.)

To go to the Gateway, click on the Gateway Overview menu at the top.
There you have 3 options. #1 and #3 are options that PayJunction offers without the need for PHP PayJunction or MO PayJunction. However, they do not look as professional as Option #2.
Option #2 looks like this:

As you can see, there are 2 lines under Quick API Connections.
In order for your website to work properly, the Gateway API QuickLink must be turned On (first column,) and of course, it has to be present. If not present, you were not given the old gateway access. Instead, look below and edit your account settings.
The MO PayJunction interface asks you for a log in and password, the log in will be what appears under Api Login. If it is not On, check the next chapter below. The Security link takes you to the security settings of that entry. Make sure you have the CVV/CVC2 turned On otherwise it is not likely that credit cards will be accepted.
You can also setup the AVS. If you are receiving orders from all over the US, Zip code and Address will work just fine. If you are receiving orders from outside of the US, the Address checks will generally fail and your transactions will be refused. In that case, require just the Zip code. With PayJunction, you can also select to match Address or Zip.
With a newer account, you will want to setup a Gateway in your account, or someone else account. You can also create a new account under the Settings menu. Then add the QuickLink API Access to that account. I do not recommend that you use your main account to create the gateway, although since they ask you for a different log in name, it is definitively safe to do so.

You can see the New User button at the top right corner. If you already have an account, use The Update Settings to modify the settings.
Part of the account settings includes 3 gateways. Each have an On and Off selection. By default, all those features are Off. Select On for the QuickLink API Access, enter a log in and a password, and click Save at the bottom.
The Reset Password and Send Email check box can be used to reset your password in case you lose it.
The log in can be as random as your password to make it much harder for a hacker to find it. Your log in name will never be visible by the customers. Make the log in as long as you can.
Enter many letters and numbers in your passwords. Use upper- and lower-case letters. To make it much safer, enter as many characters as you can.