The official soccer rules are called the "Laws of the Game" and are revised annually (usually in July) by FIFA (pronounced "FEE' fuh"), 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. You can find the "Laws of the Game" at FIFA.com (on the FIFA Home Page enter "Laws" in the search feature).