Below are a list of documents and websites that are worth investigating to improve your referee skills:

The Laws of the Game (LotG)

2016-17 IFAB Laws of the Game – English
2016-17 IFAB Laws of the Game – Spanish
2016-17 LOTG Changes Summary

The Referee section of FIFA.com

FIFA Refereeing

IFAB

The Laws of the Game and related documents

The Referee section of USSF

USSF Refereeing

The Referee section of AYSO

AYSO Refereeing

The AYSO Basic Referee Online Training course (BROT)

AYSO Training

Ask The Ref – Where games scenarios are presented for different referees to explain (and other information as well)

http://www.asktheref.com/

Ten things a ref can do to prepare for the season

Below are documents worth reading (and are the most current versions):

2015 AYSO Guidance for Referees, Coaches, Volunteers & Parents
USSF Guide To Procedures – English or Spanish
2015 AYSO Manual For Youth Referees
USSF Information For New Referees
Requirements for Injured Players