Know the Rules

The big three disciplines to explore are IDPA, USPSA and PCSL.  Steel Challenge is also a fun multiplatform firearms discipline that is a great way to develop action shooting skills.   Each of their respective websites contain a published rule book.  Skim each one and get to know the general rules.  Attend matches and learn from observing.  Learn from connecting with your new community of competitors.  

Above all, you absolutely must know the basic safety rules and commands.

International Defensive Pistol Association - IDPA.com

United States Practical Shooting Association - USPSA.com

Practical Competitive Shooting League - PCSL.com

Steel Challenge Shooting Association - scsa.org/rules