This is how you play Shen as a tank. A pretty strait forward build on a basic tank.

As far as farming, do it a lot, like the usual. Farming early game with Q is quite usefull, and when low health, make sure to not just farm, but also lifesteal with your Q. Get into the habbit of spamming your Q. It will help you a lot in farming, and lifestealing with Q. As far as how you can poke, farm normally, and when enemy get too close, then Dash to them with E, and attack with Q. Since Shen is a solo top (Usually) poking wont get you much damage on enemy. If anyone under your turret, make sure to use your E to your advantage, it forces them to attack you under turret.

As far as how to team fight, make sure you Q enemy champions, in order for your team to heal off of them. It may be a very little heal, but still it will help. Also, make sure that you are tanking, as in you are the tank, so E into your enemy's face and let your ADC do massive damage. Simple enough?

This I found is a very hard thing to get used to. You need to have full map awareness. This means you will have to pay attention to other lanes more than you do to yours. As far as when and how to ult, I either ult when ally champion is getting low, and as soon as you get out of your ult, make sure to E them into the face, this way it will get them off your ally.