First you will need Java Runtime Environment (JRE). If you have a Mac, you should already have this. If you have a PC, you may not. It can be downloaded from this site. Select Java Runtime Environment (JRE) 5.0 Update 12. This is not a full development kit, so with it you can't compile a program from the command line. For that, you should download JDK 5.0 Update 12 instead.
Then you can download Eclipse from this site. Click on Eclipse Classic.