Telnet Help!

[Kipsta]

Okay I have some little experience will keyloggers. I just do it to friends as practical jokes. I was wondering if there was any way to get into a computer without the person knowing and keylogg them. I have been experimenting with telnet and I tried connected but it would never connect because the port I put in is wrong. HOW DO I KNOW WHAT PORT TO PUT TO HACK INTO SOMEONES COMPUTER? I have tried 80, 25, 2* .... and nothing works!
Please help me! :) thank you