Do some searching.
You have to decide if you want to add internally or externally.
The second alternative, external, is far easier. Just buy an external, hook it to your Mac with the appropriate cable, run Disk Utility and format it to HFS+ journaled. Then use it for storage, backups, Time Machine, etc. It acts just like a giant version of all those volumes you've seen pop up from .dmg files you're downloaded.
Internal is tougher; check out ifixit.com to see what you're getting into. But you can, as suggested, add a SSD drive, which speeds performance of many tasks considerably.
You can also partition the physical drive, internal or external, into one or more volumes. And one of those could be used to hold a clone of your system. That's very helpful if the other drive fails; you then boot up from the clone and keep working. You can also set up a different system on that other hard drive; lots of folks do that with Snow Leopard if it runs on their machine. Then they keep Lion on the other drive. You can choose which to boot into with Startup Disk in Prefs, or by holding down the option key when you start up.
It's a good idea you have, you just need to let us know what your needs are so we can suggest which of all these opportunities might help you the most. E.g., you might need lots of storage for media, but have another computer. So you don't need a clone. Or the opposite.
Rob
