How do I set up "Time Machine"

I am on http://support.apple.com/kb/HT1427, but the information given on how to set up "Time Machine" doesn't match with the prompts my Mac mini is giving me.

The support site says I will be asked,

But I don't get this ^ prompt. The prompt I get says "Choose a backup disc." And when I click on "CHOOSE BACKUP DISC" (my only choice) it brings down another drop down menu and my only two choices (prompts) are "set up time capsule" and "cancel." When I click "set up time capsule" my computer searches for an "air port utility" It then tells me I don't have one and then I am stumped. I don't want to install "Time Machine" with an "air port utility." (I don't even know what an air port utility is.) I want to use an external hard drive I bought so that I'd have more space as my backup disc.

Any suggestions on how I can install Time Machine? My Mac Mini is 2 and a half years old, but I just recently started to use it as my "default" computer.