I seem like a sitting duck in team fights, I don't get what I'm supposed to do. What I generally try to do is ult my carry when they're taking damage then just keep taunting the people who are doing too much damage to them, but after my taunt is on cooldown and I'm out of energy I feel useless. 
Try maxing your shield second. If you build right you should be able to get their team to focus you by going after the carry and survive while your team does their jobs. Save your taunt for later in the fight to peel for your team if you lose and to make any stragglers stay a bit longer if you win.
I mean, that's basically Shen's fatal flaw as a tank; he doesn't do enough to make the enemy team pay attention to him once his taunt is down. I guess the best thing to do is just soak damage, dive the enemy carries, peel for you carry, and also Q whoever your ADC is attacking, that way they heal for more (great for countering a thornmail+gromp jungler for instance).
I've seen one guy do trinity force then build full tank. Trust me, he was super annoying to deal with. He did decent damage while having really good defense to back his team up.
Use your Q on whoever your team is focusing
Try to use your taunt on as many enemies as possible
Use your ult at the right time, especially on your carry after an engage so you can follow up with a taunt
Building one damage item on him usually makes him harder to ignore, I've seen 
or 
on tanks like shen who are pretty ignoreable after their long cd cc is down, I think frozen mallet is a pretty decent pickup, it applies a 35% slow on every auto, which helps you peel for your carries more effectively.
ult the strongest carry on your team (if your mid is burst its gonna be your ADC) then when you land taunt the enemy adc and anyone close to the person you ulted. gg youre done for the fight besides throwing a q and autoing some stuff.
I like to initiate with taunt on enemy carry and whoever else I can get. Tank enemy until your team joins then gtfo to the side and use ult on your carry and rejoin fight. Full tank Shen does plenty of damage if you get ahead. The trick to Shen is making good use of his map control.
i like tp smite shen hes really good if you get your cinderhulk and thorn mail god bless after that.
Focus carries, hard engage for your team. Pretty much it.
