Below is a basic outline for various soccer groups.  There are a lot of resources out there that cover each of these in detail.  This is just an initial information collection for you!

U4
Players are turning 3 beginning in the fall season
Typically 4v4
Can be coach reffed
Small goals (often popups)
Substitution only at quarters unless there is an injury
Size 3 ball
6 minute quarters

U5/6, U7/8
U5/6 Players are turning 4 and 5 during/just before the fall season.  U7/8 are turning 6 and 7 during/just before the fall season.
4v4
No offside
No keeper
No intentional headers (indirect free kick if happens)
Corner kicks
Goal kicks
All kicks from fouls are indirect
Throw in - Both feet remain on ground, ball must come completely over the head and release happens once over the head, but cannot "spike" ball (refs will often allow retakes on illegal throws)
Substitution only at quarters unless there is an injury
Size 3 ball
8 minute quarters

U9/10
Players are turning 8 and 9 during/just before the fall season
7v7
Offside with build out line (between build out line and midfield there is no offside)
Keeper
Keeper cannot drop kick/punt and must roll or throw the ball into play
No intentional headers (indirect free kick if happens)
Corner kicks
Goal kicks
On all goal kick and Keeper restarts, opposing team must move beyond the build out line
Direct kicks on relevant fouls, including penalty kicks
Throw in - Both feet remain on ground, ball must come completely over the head and release happens once over the head, but cannot "spike" ball (refs will most likely not allow retakes on throw in)
Substitutions only at goal kicks, your team's throw in, when opposing team is subbing, or at the referee's discretion
Size 4 ball
25 minute halves

U11/12
Players are turning 10 and 11 during/just before the fall season
9v9
Offside (no build out line)
No intentional headers (indirect free kick if happens)
Ball must visibly move on restart by kick
Other rules from U9/10 apply
Size 4 ball
30 minute halves

U13+
Players turning 12 and up
11v11
Headers allowed
All other rules for U11/12 apply
Size 5 ball
U13/14 - 35 minute halves
U15/16 - 40 minute halves
U17+ - 45 minute halves