The official soccer rules are called the 'Laws of the Game' and are revised annually (usually in July) by FIFA (pronounced 'FEE' fun'), the world soccer governing body, but youth organizations usually adjust the rules to fit children. Typical adjustments are field sizes, game lengths, number of players per team, the number and frequency of substitutions, 'offside' is sometimes not called, and slide tackling is sometimes not allowed. Field sizes, ball sizes, length of games & rules vary by age group. The FIFA rules do not require separate teams for girls and boys, but many soccer clubs and associations have separate leagues for boys and girls.
Discuss the rule variations with an official of your league. In brief, the 'Laws of the Game' are not simple and can be difficult to understand. The official FIFA rules have 17 sections and the rule book is about 70 pages long. There are also an additional 44 pages titled 'Questions and Answers'. In addition, how to interpret the rules is discussed in referee clinics, special memos, videos for referees, and a guidebook for soccer officials that is over 300 pages long. There is also the problem that the rules use many soccer terms that are not defined in the rules (we try to define all of these terms in the Soccer Help Dictionary) and there are quirks, such as the fact that terms which are commonly used such as 'Hand Ball', 'obstruction', and 'Linesmen' are not defined in the official rules.
Another confusing aspect of the rules is the way 'Fouls' are defined; basically, they are defined not only as 'Fouls' but also in the rules regarding 'Cards'. For this reason, to understand 'Fouls' you must also read 'Cards'. One of the biggest problems is that the official rules don't have a good index and there aren't good cross-links to related topics. At Soccer Help we have tried to solve these problems by simplifying the rules and providing easy to use the links so you can go from one topic to another. In some cases, such as for 'offside', we have even provided a simplified explanation and a more detailed definition.