How can you tell?
Knowing whether your computer is MIDI capable or not is pretty simple and straight forward. You're basically going to look at your computer (back and front) in search of one of two kinds of ports: MIDI or USB. There are pictures of both ports in this tutorial. If you find one, read on. If you don't find at least a USB port, you should consider upgrading your computer. Either buy a new one, or buy a USB PCI card. If you need help with that, visit my profile, and use the contact form to shoot me an e-mail.
Sound Cards
Probably the most common place for a midi port is on your sound card. Here is what a midi port looks like:
This one is on a E-MU 1820, which I use to connect my ToneLab.
MIDI ports are often found on all pro-audio sound cards, and many higher end consumer sound cards. If your sound card doesn't have MIDI ports, don't panic. There are other ways.
USB/MIDI
A good, universal way is to get a USB/MIDI adapter like this M-Audio USB Uno:
Of course you'll need a free USB port like this one:
This should work with any computer that has a USB port, and you can pick one up for about $40.00.