Give your player an instance variable (called CriticalChance ?) and give it a value (1 ?) then you can put something like ..
if player hits then damage = 10
if random(100)<chriticalchance then damage=damage *2
take damage from baddies hitpoints
?
as player puts experience into critical chance (player goes up a level)
make (variable) CriticalChance =CriticalChance+1 (or 5 or 10)
?