Rocke Verser's DES Challenge homepage has all the information you need to get started. You can also read about RSA Data Security's challenge.
To help out, all you need is a Mac, Win95/NT, OS/2, or a Unix machine. The client software runs on almost anything. It runs in the background only using spare CPU cycles. The program is also very small, so the load on system resources is not noticable at all. Any spare CPU cycles being used gets us closer to finding the key.
Client are available for a huge list of platforms: PC running almost any OS, Macs, and nearly every modern Unix. The best place to go for the software is here. It is fully automated, and has the most recent clients.
If you've got an NT or '95 machine, you might want to check out the DES GUI Front-End program. It has a nice interface and provides a great deal of extra functionality. More info can be found here.
Once you have the client, run it using keymaster.verser.frii.com as the server. Or you can use the UIUC UDP relay service at locutus.cso.uiuc.edu. Either way you will be joining everyone else using Rocke's server.
If you are from the U of I, please register your machine by sending us email at email@example.com. If you don't want to be included in the list of people involved, just say so.
You can also join the U of I DES challenge mailing list by sending "subscribe desUI" in the body of an email message to "firstname.lastname@example.org"