There are several ways to increase your critical hit %....
- Increase your dexterity. 10 points of dexterity gives 1% chance of critical hit
- use a weapon that boosts critical hit %. My bow gives an additional 6%.
- when you increase in levels, you are given skill points that you can apply to directly increasing your critical hit % (if you are a ranger or mystic)
- All that being totalled, the max % applied is limited to 50%
I suggest looking at higher level players of your type (mystic, warrior, ranger) and see what combinations of equipment, specializations and skills they have. It can give you ideas on how to develop.
Hope that helps.