Bath Rugby Pirateers :
Win games, build bigger stadiums :). Friendlies don't count, games have to be league, tournament or GC's
You can also increase your fans by using your 5 training matches a day and winning. If you lose a training match you don't lose fans but you don't gain any either.
Start by increasing your stadium, then play the already mentioned games (training, tournament, gc's and competition).
Good luck.