If you want an answer to a rules question right away try the judge chat! 24/7 rules Q&A with certified judges.
Our FAQ is actually just a copy-paste of the original on the Judge Wiki!
If you really want to delve into the minutia of the rules the Comprehensive Rules are your final stop. The parts that cover Two-Headed Giant are 805 and 810.
Use this tool to find a 2HG prerelease near you! (Endorsed by everyone’s favorite Comprehensive Rules entry, 100.6b)
If reading about all these rules has wet your appetite and you’re wondering whether judging is something for you, this is a good place to start.