(aka "Subbing"). Youth Leagues usually either allow "unlimited substitutions" (which usually means the coach can "sub" as many times as he wants during the game but only at certain times such as goal kicks) or only allow subbing between quarters. If "unlimited substitution" is allowed, you can
usually sub at these times (check with your league to see if they follow these rules): after a goal kick is called for either team, after a goal by either team, after a throw-in is called for your team (
not the other team), at halftime, and at an injury time-out if the other team replaces a player (but you can only sub as many players as they do). You usually cannot sub on corners, or free kicks. Except at half-time or between quarters, substitutions may only occur with the Referees permission (you can get his attention by yelling "sub"). Players entering & leaving the field should only do so at the halfway line. The rules technically say that a player must leave the field first before his sub can enter the field. Many referees don't enforce this in youth games because there is so much substitution. However, if the Ref says "call them off first", this is what he means. Often, midfielders are subbed the most because they run the most.
See Substituting (Substitution, Subbing) - Rules, When To and How To