First you must know the stages of critical. There are 5 stages and each stage increases your percentage of chance of landing a critical.
Stage 1: 6.25% (1/16)
Stage 2: 12.5% (1/8)
Stage 3: 25% (1/4)
Stage 4: 33.3% (1/3)
Stage 5: 50% (1/2)
When you use a move that has a high critical hit ratio it increases your chance by 1 stage. So for example you use a normal move you will have a 6.25% chance of landing a critical but if you use a high critical move your ratio will increase to 12.5%.
Here's some ways to increase your critical ratio.
There are 14 moves that increase your ratio and these are:
Aeroblast, Air Cutter, Attack Order, Cross Chop, Cross Poison, Leaf Blade,
Night Slash, Psycho Cut, Razor Leaf, Shadow Claw, Slash, Spacial Rend, Stone edge and Focus Energy (focus energy raises your critical ratio by 1 stage only for 1 turn).
There are also 4 hold items that increase your critical ratio:
Lucky Punch (exclusive to Chansey)
Stick (exclusive to Farfetch'd)
There is an ability that doubles your crit ratio, Super Luck. For example your crit stage is on 2 because of super luck it increases it to stage 4.