@lord berte
Leagues: Ranking in leagues determined by the number of Yards you acquire. These can be roughly measured by the number of Touch Downs you get. Naturally the more touchdowns you've completed the more yards you'd have done - Roughly there's 80 yards per touchdown
You get touchdowns by passing to players which look something like this:
Each player will progress across the field by a different amount and have a different cost. You can get a rough idea how much they progress by the markers on the ground.
In this example you can see the amounts are (from the 22nd marker facing to the right - we're going from the marker the football itself is touching):
57 footballs = 26 (4 yards)
88 footballs = 28 (6 yards)
108 footballs = 29 (7 yards)
You can hover over (or click on if using mobile) each player to see what prizes they give:
Reading the League Table
As stated earlier there's 80-ish yards per touchdown. So if I want to get into the Bronze League I need another 6 touchdowns
NOTE: Whenever you or anyone else progress on the field, the scores needed change. Notice how the rewards are separated by "Top x%". This means only the top 1% of players will receive gold.
Fully completing all event quests and using all free currency earned from completing quests should give you a reasonable chance at getting into the Bronze league. Many players don't fully complete the event and these are the players that'll be in the Rookie or Amateur divisions. The Silver league in previous events could be achieved without spending diamonds by a few players - but would require you to have particularly good luck in what yardage options the game provides.
To increase your chances of getting into Silver you can go into the forum competitions to win free event currency anytime and event is on. Each servers forums are run independently from each other so the prizes available will differ depending on which server your game is being played on. Some players have managed to get into Gold simply from the free extra currency won from forum competitions - particularly the competitions that have no limit on the number of winners.
However the bulk of those in Silver and Gold leagues (those that push the numbers up as high as the 1257 figure) would have to spend real world money to do so. The figures you see there would most likely only be the absolute bottom player of each percentage bracket given that's the minimum amount needed to achieve that ranking. I wouldn't be worried about aiming for any specific league, but rather aim for the prizes you want when spending your (free) footballs ignoring the league results.