How to calm down? Listen to this http://youtu.be/Po8rG4nXpUs It's not the end of the world. You don't need to do this all perfectly. Just enjoy the date and stop trying to make it be something.
What to wear? Depends on where you're going. I always advise wearing a red, blue, or green. Dress up to the point that it's noticeable that you tried and were very excited about the date but don't dress up so much that you'll risk looking out of place or that you're uncomfortable.
How to act? Don't act. Acting on a date almost always leads to everything feeling fake and disingenuous. Instead of acting do what I do and focus almost all of your attention on two things appreciation and curiosity.
What to say? Be kind to everyone not just your date. Ask questions and not like surface level how's the whether kind of stuff. Try digging a little deeper. Don't be afraid to say what you like and what you believe. No one wants to date a liar.
"don't say be yourself" so who are you going to be? if he likes you, he likes you. if not, he's not. but if you pretend to be something you aren't, then what are you trying to do? at some point, he's gonna se "who you really are". anyways, you didn't give us any information. where are you going? when are you going? you aren't going to wear a dress if you go to an amusement park, for example. and it's just meeting up with a dude, you aren't at a job interview. if you were meeting a friend or someone you weren't attracted to, you wouldn't think 2 shits about it. also, he's a business man. maybe at 30, he's a little more sophisticated at times, but i was a business major too... most of us are clowns. he's just a dude. anyways, good luck :D