Different rates apply. How much of your credit is used up will depend on where you are calling to.
Skype rates page
If you don't have a Skype subscription you will need to concentrate on the Pay As You Go (PAYG) rates.
I choose the pay on the way or monthly subcription, the next step always make me choose this one "Discover new features with just a little Skype Credit. $10.00" Could you tell me what this for.
Even when using Pay As You Go, you have to buy Skype credit ahead of time usually in 10 or 25 dollar increments. So you make a 10 dollar commitment which acts as a buffer to handle the calls you are going to make (deducting accordingly). By default Auto Charge will be checked. If you do not want to be charged another 10 dollars when your credit goes down to 2 dollars, you will need to uncheck it. Should you need more minutes in the future, you will need to purchase another $10 dollar block of credit.