Do you mean make the free video calls or set up an account to call phone numbers?
If it is the first opton, you need to search for your friends and add them to your contact list. Skype allows to search by email addresses as well as skype IDs and real names.
If you want to use it to make phone calls, you need to register a debit card and you can choose between paying monthly or pay-as-you-go style. Then you can dial numbers on the keypad like a regular telephone.