This should be in the help...
But I'll tell ya.
I kinda just went around killing stuff in the towns, since in KH 2 the boss aren't that hard. like spot wise though depending where you are at, the peak in pride rock, the underground in twilight town, The final level in the castle, or Hollow Bastion (Or You know what.... don't really wanna spoil anything). An alternative is the tower where Yen Sid is, or whatever his name is. The whizard guy. Tron's world... I don't wanna give too much away >.<