I do the same on my servers (only you can’t buy admin, that is a horrible idea). First you’d need a payment method (PayPal, Google checkout, etc.)
Then you need to learn how to use the API for that method. PayPal’s is pretty horrible, but it works. Then make a PHP script to enter data (such as SteamID, transaction token, email, etc.) into a database, and another script for when the PayPal process is complete to grab the data (token). Grab the data from PayPal, perform the payment process, and then mark the transaction as complete. In game, you’d need to connect to the database, lookup the SteamID and check for a completed transaction.
It’s not really something someone can write a script for you for, and no one here probably has time to explain it fully, but lookup PHP, databases and the PayPal API. Oh and you probably need a premium PayPal account, too.
I’m fairly experienced with PHP, and even this took me a while. If you’re not “that great” at scripting, you should probably look into other methods (or be prepared to spend a while coding it)