some Skype services such as Skype-to-Skype calls, conference calls, chat and group chat, 1-on-1 video calls are indeed free of charge. However, some services such as Skype-to-mobile/landline number, group video calls, voicemail, SMS, online number, etc requires credits or subscriptions.
The support/user guide section offers a mode detailed descriptions - https://support.skype.com/en/
Moreover, you need to install Skype first in your computer or device. First go to the downloads section of Skype.com (http://www.skype.com/en/download-skype/) and download the installer for your computer or mobile device. Then after installing it, you can use the basic and advance features of Skype such as calls, video calls, chat, etc, etc.
Visiting the user guide section might also help -