If you look at a football game and how they play say 4 ,4,3 or 5,4,2 then basically on most fields in 7 man you would be playing say 3,2,2 .
3 at the back (both corners and 1 centre ) 2 mids could be 1 on the left and right (depends on field lay out) and then you will have your 2 fronts same again 1 on the left and right .Read all the p8nt classroom stuff and anything else you can get your hands on,also watch as many games as possible it gives you an unbelievable insight and depth of the game and how it can be played .