I use Skype quite a lot but not for telephoning. Quite few of my friends also have it installed and I've added them to my contacts list.
You need also some headphones with a microphone boom, OR as I do, a webcam, with built-in microphone plus external speakers.
The webcam I use is the Logitech Pro 9000.
To add a contact you can ask your friends to let you know their 'Skype' id, or just enter their email address in the search box. Give them a call by clicking on the round green handset icon and leave a message if they do not answer. When they next log on, they will see that you've called and , hopefully, respond accordingly. All the above costs nothing.
I was curious to see you ask 'How do I buy credit for it' as you stated in your original post that you did buy ten Euros worth.