For now, please see this (rather detailed!) handbook regarding security:
Activist Security Handbook (pdf)
Plus, the same collective have also produced this guide to meeting up in public places:
Pub Meetings (pdf)
This website also provides some information regarding internet security and for those who understand french, there is also this one. The Tactical Technology Collective also provide some useful information and resources.
A basic guide to computer security can also be downloaded here: Computer Security (pdf)