Unfortunately you can't make Facetime to Skype calls or Skype to Facetime calls. The reason is that, their are entirely different applications.
Facetime is from Apple and Skype is from Microsoft. They both use different environments. You can only make Facetime to Facetime or Skype to Skype calls.
If you want to call someone on Skype, you have to download and install Skype on your Mac. You can download and install it from this website --> http://www.skype.com/en/download-skype/skype-for-mac/
After downloading Skype, open it and open Skype application. Then create an account from there and you can find your Skype contact by entering their name and make calls.
If you have any difficulty on following the steps, please let me know and I will walk you through it.
This is not my final answer. I will continue to help you until you are 100% satisfied.
If you want to continue later, then add a bookmark of this page or add this page to your favorites, so you can get back to me any time you want. This question will not be closed.
If you want, I can even remotely connect to your Mac with your permission and setup Skype for you while you watch.
Hi there, Its me Ashik again. I'm just following up with you. How is it going ? Were you able to install Skype ?