Below is a link to a soccer position diagram.
Until U8 coaches don't need to worry much about soccer positions. At SoccerHelp we don't recommend that U4 and U6 players play real matches. Instead, we recommend a once-a-week session for about 45 minutes where they play our soccer drills that are games. We got the idea from LNYSA in CA and call it "Soccer Fun for Ages 3, 4, 5 and 6". It is easy to administer since it doesn't require refs, uniforms, schedules and fields and is very popular.
At U8 and older, I think it is important to start teaching players the basics of soccer positions. Most coaches will start to teach positions because the alternative is "herd ball", and you don't want to allow "herd ball" because that is a bad habit to break. How to teach soccer positions and soccer position basics are discussed in detail on SoccerHelp.
There is over 100 pages on SoccerHelp about soccer positions and a diagram that shows the positions.
This link will take you to a Soccer Position Diagram and Soccer Positions Basics
There are diagrams on SoccerHelp that show soccer positions on kick-offs and when attacking and defending.