Skype is free, although I believe you can buy credit to call landlines if you wish to do so.
You need to download an app or program, whichever suits you best whether you're on a computer or tablet.
You will have to download the program, sign up and log in, as will the person you are wishing to talk to. You exchange your Skype email or username.
You will need a camera and microphone, sometimes they are as one, sometimes the are separate pieces of hardware.
After you have logged in to Skype, you will see a list of your contacts. Simply click on the contact and choose whether to video call them, voice only call them or simply text chat, where only a keyboard is required.
It is FREE. You will not be charged unless you choose to be charged. You can chat away as long as you want BUT the quality may degrade after a while, at least that has been my experience.
Skype was recently bought by Microsoft, and I haven't really used it since, so some of my information my be incorrect but hopefully it hasn't changed so much so that my words will be completely useless!