Before you start:
Run the MTA client, and tell us what it says up top.
If it says "GTA3: Multi Theft Auto", then skip everything below, and tell us that it says "GTA3: Multi Theft Auto".
If it says "MTA:VC - Multi Theft Auto - http://www.multitheftauto.com", then read what's below, and Then tell us that it says "MTA:VC - Multi Theft Auto - http://www.multitheftauto.com"
Which patch? The unofficial 1.1 one?
um... if you're trying to play MTA, then you don't need to run the GTAt thing. If you're trying to play GTAt, then please head to their forums
This is in the GTAt client... correct?
you aren't supposed to enter your own IP (what you're calling a host) and port... You have to enter the IP and port of a server. You can find servers using the All Seeing Eye (check the 'first time MTA user guide' that IJs linked to in his post)
It doesn't yet have the option to let you choose between GTA3 and GTA:VC...
you don't need to know what your host (better known as IP) is.