Mainly, what people should i target with it? I like to hit the ap carry to prevent them fro using abilities, then, if possible, the ad carry to make them hit me, then the support to prevent heals. Is this right or am i clueless?
You want to try and hit as many as possible basically.
You have the correct idea though, prioritizing those targets.
You will almost always want to hit the AD carry as they generally have the most DPS and potential to wreck your team. Taunting them takes them out of the fight for a short while. The only time you wouldn't target them is of course if they aren't there, and if they are severely underfed or under-leveled.
Targeting the AP carry is also a good idea as like you said, it keeps them from casting spells. You will especially want to target AP carrys with channeled abilities, especially Katarina. This stops the abilities and in turn most of their damage.
Targeting supports isn't a high priority due to their low damage. Unless they built items for damage or have great team-fight abilities (Sona and Nami's ult for example) you might want to target them and get your team to focus them down before they get the chance to use their ultimate's.
In the end though, the general idea is to target as many high-damage / high-utility targets as possible.
Shen's taunt has 3 basic uses: initiate, protect, and escape.
Escape: Shen can use his taunt to quickly put distance between himself and his opponents or to hop over small walls. This is useful when escaping from ganks or chasing through the jungle.
Initiate: Shen's taunt is very useful to start teamfights. By forcing opponents to attack him for 1.5 seconds, he gives his team an immediate advantage. It's important to hit as many champions as possible when doing this, especially carries.
Protect: If you find the opponents bruisers targeting your carries, you can use taunt to peel them away and protect your carries. Due to the somewhat long cooldown (16/14/12/10/8), it's not always possible to use this twice in a fight, forcing you to pick between initiating the fight or peeling for your carries.
If you find yourself pushed under a turret with an aggressive opponent, you can taunt them to force the turret to attack them.
Choosing how and when to use taunt can be a difficult decision. If you have another strong initiater on your team (ex: Amumu), it may be better to defend your carries. However, if you're the tankiest member of a team of glass cannons, 1.5 seconds may be all they need to blow the enemy team up.
P.S. Taunt costs 120 energy and restores 40 energy for each champion hit. Hitting 3+ champions will completely refill your energy, while dashing away and hitting no-one leaves you very low. It's generally a good idea to shield before using taunt as well.