The standard utility to do that is VNC. There are many VNC servers and clients; this Web page is Ubuntu's guide to it. You can download onto your father's pc one of the servers as follows:
sudo apt-get install vnc4server
A good install guide can be found here You will then need a client on your pc,
sudo apt-get install xvnc4viewer
Its use is quite straightforward. Now, when you connect, you will have your father's pc right in front of you, as if you were sitting at his chair.
Two more small issues:
If you wish to connect to your father's pc even when you are away (at work, on holiday and what not), you will have to do some port forwarding, i.e., to instruct your router that, whenever communication arrives on port 5901 (the port generally used by vnc servers), they should be addressed to your father's pc;
Again if you are considering connecting to your father's pc from outside your LAN, you should consider encryption. The normal communication set up by VNC is plain, i.e., any snooper can listen on onto your conversations. If you wish to defend yourself against this possibility, the standard solution is to first establish an SSH communication (which is encrypted), and then vehicule your VNC session over the ssh channel, which makes it automatically encrypted. For Ubuntu, you find instructions here