Apologies but this is a bit of an infodump. There unfortunately isn't a 'basics' version of reffing, you will eventually need to know it all, this will take time so don't panic and think you need to know it all by next week. I've still got a long way to go to be anywhere close to up to scratch. 

Bear in mind for all of this that Derby as a serious sport is still in its infancy, the rules undergo revisions frequently and even the wording of the rules sparks debates on how to interpret them. Expect to be told something slightly different every few months.  

Rules Etc.

This is the current version of the WFTDA rulebook, its fairly densely worded, but unfortunatley you'll need to know it all to ref effectively. You won't understand it all first read, some of it makes no sense until you see it in action.
http://wftda.com/rules/wftda-rules.pdf

Hand signals, you'll need to know these well enough to do them quickly and accurately

http://wftda.com/rules/wftda-rules-appendix-c-referee-hand-signals.pdf

Standard practices, basically a guide on the best ways to staff a bout, reffing positions and procedures

http://wftda.com/rules/wftda-officiating-standardized-practices-2010.pdf

Minimum standards, not compulsory for refs, but they really should be

http://wftda.com/rules/wftda-minimum-skill-requirements.pdf

Reffing positions & Responsibilities

Head Ref

Positioned in the centre of the track, acts as a final arbiter of decisions and generally runs the whole thing and also acts as an additional pack ref.

Pack Refs

Two inside the track and Three outside. Pack refs monitor action in and around the pack, declare the pack position and no pack situations, call penalties by blockers and generaly ensure fair play by the non jammer players. 

Jam Refs

One per team, following one jammer each. Jam refs keep pace with their jammers, declaring if they are lead jammer or not, keeping score and comunicating score to the score keepers, calling penalties on or by their jammer and calling the end of the jam if their jammer signals it. 

NSOs

Non skating officials, responsible for the 'admin' side of running a bout, including penalty trackers, penalty box timers, bout timers & score keepers.

Other Stuff

http://www.zebrahuddle.com/ 

an excellent Refs forum, good for rules questions, gear opinons and hypothetical discussions.

http://www.justin.tv/derbynewsnetwork/videos

DNNs video feed, one of the best ways to learn is to watch :)

Welcome to Team Zebra.