There is plenty of resources on the internet, you will need to know assembly language and there are some great tutorial series on that here http://www.securitytube.net/groups?operation=viewall&groupId=0

Research tools such as gdb, ollydbg and ida pro.

This step by step tutorial may also help
http://resources.infosecinstitute.com/step-by-step-tutorial-on-reverse-engineering-malware-the-zeroaccessmaxsmiscer-crimeware-rootkit/