I've pretty much built all of my externals.
1-Go to a computer or electronics store and buy a Hard Drive case. There will be a few choices, USB 2 and Firwire and some that will do both. Buy what will meet your needs best. A good factor to think about: Which do you have more open ports of on your Mac, USB2 or FW? Make sure you know if the external case is for ATA/IDE drives or SATA drives.
2-Find a deal or sale on a big IDE or SATA hard drive, depending which you bought a case for.
3-Put the drive in the case and plug the FW or USB 2 connector into your Mac.
4-One of two things will happen. Either the drive will just mount on your desktop or OSX will tell you and unformatted drive has been inserted and ask you what you want to do with it. Click Format and OSX will open up Disc Utility. If you are only going to use the Drive on a Mac, select Mac HFS Plus (Journaled). If the drive will be used with a Windows machine as well, select MS-DOS and click format.
5-Once the drive has mounted on your desktop, use it.
I don’t know about Bus powered drives, none of mine are.