This is a common problem getting Remote Desktop to connect to your PC, there can be many causes.
- Make sure that your version of Windows supports remote desktop, Windows Home editions don't.
- Make sure that remote desktop is enabled. Press the start menu and type "Allow remote access to my computer"
- If this has been enabled then please try to connect via WIFI first
- If WIFI works but you can't connect from outside your home/office then you need to setup port forwarding on your router. Please take a look at this website for instructions for your router: https://portforward.com/
- Once this is done you should be able to connect using your external IP address (eg 126.96.36.199 in this example). You can find this by visiting whatsmyip.org.
- You can also use services like dyn dns which can give you a URL such as myserver.dyndns-server.com. Your router might support updating this with your IP address automatically when it changes.