First of all, congratulations on your work to become a Magic Judge!
Inform your local player community
The first thing you’ll want to do is to inform your local stores and players about your advancement, so they can ask you whenever they have questions. You can still play Regular REL tournaments and act as a judge at the same time, helping out players at tournament. You can still play higher REL tournaments, as long as you don’t play and judge the same tournament.
You can find your local stores using the Wizards Event Locator.
Contact your local judge community
You also want to contact your local judge group, and interact with them regularly. You will be able to find out updates and changes with them, and help each other to be better judges. Your Regional Coordinator has most regional information which you are looking for. If you have suggestions on how to improve Magic: The Gathering™ for your local community, or ideas and suggestions on the program, your Regional Coordinator can also help you on those, you can contact your Regional Coordinator using this form. Also check out the list of local judge sites and mailing lists.
Next step: Competitive REL events
As a judge, stores may want your help on some Competitive REL events, assisting at a PTQ or other local Competitive event. You will need some extra study for those events, as they are no longer just for fun, but for larger prizes, invitations or byes for larger events. The Magic Infraction Procedure Guide is a document that tells judges how to handle issues and which penalty corresponds for tournament and play errors, as well as how to fix them. This document can be found at the WPN site. Translated versions of these documents to many languages can be found at the Translated Rules blog.
There are very useful tools that will help you as a judge:
In JudgeApps you can find forums when seeking help from judges of various levels on various topics. Judge projects are managed from here, and judge applications for major events (including Pro Tour Qualifiers, World Magic Cup Qualifiers, and Grand Prix events) and handled here. Reviews and investigations can be entered now on JudgeApps.
Want to talk to other judges online? Have questions that need immediate answers? Join the IRC judge channel! Get instant access through the Magic Judges web interface. Alternatively get an IRC client and find the #magicjudges-rules and #magicjudges-policy channels on Freenode. Windows users might want to use mIRC. Find more information at Updates to Judge IRC Channels on the JudgeApps blog.