I need your help!! I download and installed Trillian but have no idea how to set it up and use it.

First question is: Do I have to have accounts from Yahoo and AIM (no way, not on my computer). Reason I am asking is, when installing Trillian, it asked me what my user name was for both of these accounts, including ICQ. I thought Trillian was supposed to just let me contact those friends of mine who use these programs. So, please dear friends....come to my aid cause I really don't see any difference if I too have to install those other messengers.