Most amateur 7v7 sides spend their first ten minutes arguing about formation. Skip that argument. Three shapes cover ninety percent of the games you'll play, and the right one depends on what your roster does best.
2-3-1: the default
Two defenders, three midfielders, one striker. Solid spine, decent width, easy to learn. If your group is mixed-skill or you don't know each other yet, start here. The single striker stays high so you always have a target ball; the three midfielders rotate based on where the play goes. Defending requires the midfielders to drop — if they don't, you'll get cut open down the middle.
3-2-1: when you have one big striker and weak full-backs
Three at the back, two in midfield, one up top — the "Christmas tree" of 7v7. Use this when you have a striker who can hold the ball under pressure (otherwise the two midfielders die alone) and full-backs who aren't comfortable defending one-on-one. The extra centre-back covers them.
2-1-2-1: when you have two fast wingers
A defensive midfielder shields the back two, two wingers/inside forwards push high, one striker stays central. This is the most attacking 7v7 shape. Only run it if you actually have the legs in midfield — the lone "6" covers a lot of grass.
How to pick on the day
Look at your six outfielders before kickoff: who's quickest, who's the best finisher, who can defend? If pace is your edge, run 2-1-2-1. If you have a target striker, run 3-2-1. Otherwise, 2-3-1.
Try it — book a pitch through Field to Play with the same six this weekend, run one shape, then switch. You'll know after twenty minutes which one your group plays best in.