Go to the dash and type in remote, click on the icon that says "desktop sharing" then click the box that says "allow others to view your desktop" on the host computer.
Now from the client computer click on the other icon that says Reminna Remote Desktop Client. Click "Create New Remote Desktop File" in the server box, type in the host computers IP address or host name, then the user name and password. Hit the connect button.
If you are trying to do this from an external computer "away from your home" you will want to make sure you have your external IP address, also you will want to open ports, or if your router supports it, you can put your computer in the DMZ (demilitarized zone) of the routers firewall.