I know what are you thinking about at the moment!
Crit Jax? Is that a real thing?
Well... it is!

Pros:
- Tons of damage
- Good lane control
- Easy to farm with
- Easy Harassment

Cons:
- Squishy
- Hard to escape from ganks (Although you can jump)
- Hard to dive with


Pros(More details):
-Tons of damage: If you bought the suggested items, then no one can ever stop you, no matter what happens.

-Good lane control: No one can stop a man(?) with a lantern. Just use you E--->Q combo, if you are done with that, start hitting them (obvious), if you couldn't kill him/her , that is fine, you can farm safely, if no one is around.

-Easy to farm with: No minion is safe from you! If you enemy is weaker, than a minion is nothing compared to you. But seriously you can easily farm with Jax, but if you are building a crit Jax, that makes it even less difficult.

-Easy Harassment: It is simple: Just use the old "Hit and Run" tactic (in this case jump-hit-run)

Cons:
-Squishy: This is his main problem, you can deal a lot of damage, but an assassin can easily put you down, if you are not paying enough attention

-Hard to escape from ganks: You can be lane dominant, but your opponent can easily kill you with the help of his/her jungler.

-Hard to dive with: Back to the squishiness! Just don't play aggressively, and don't go near the enemy tower when your minions are not around.

It is a crit Jax, so you should buy the basic crit items (You don't say??)
Yeah.. I know I should write more things about this, but I think this is obvious.

-Phantom Dancer: It is a good item in early game, but you can buy a Statikk Shiv earier, and then you can buy this

-Infinity Edge: It gives you a nice amount of damage and critical strike chance, i recommend to buy this item late game.

-Statikk Shiv: This item is a basic element of this build, you can buy it mid game and late game too, that lightning strike will help you to farm, but you can use it to kill enemies.

- Youmuu's Ghostblade: You shuold buy this item late game. It gives you increased movement speed, which will help you to your foe who tries to flee.

-Trinity Force: I recommend to buy this item late game. It gives you a lot of damage, which will help you win the game.

21/9/0 Although the power of this build lies in the damage of Jax, you still need some health to counter your squishiness.

The crit is what we are looking at in this build, but you still need damage, so you have to add those damage Quintessences to fix this issue.

You have to focus on farming first, because you need those items to kill your opponents later.
If you are having some trouble with farming, than ask you jungler for a gank, than kill your opponent,so you can farm easily.