Step 1:
Tell us what game you want to port
Step 2:
Install Apple X11, if you have not already done so
Step 3:
Install fink and see if the game you want is on there - if so, then things will be simple for you
Step 4:
Open up the terminal and look around. You won't be able to cause any permanent damage, so feel free to type things and see what happens
Step 5:
Go to a bookstore or a search engine and find yourself a beginner's guide to *nix commands, and read
If the game is not installed by this time, I will walk you through the process.