Ok, presentation is EVERYTHING. Yes, you aren't being graded on your clothes but how you look will create subconscious judgement unto yourself.
Suit: DO NOT WEAR BLACK. Black is for funerals. The most conservative colors are Charcoal, Gray and Navy blue. Go to Macy's for a suit and GET THAT SUIT TAILORED. Seriously, the best weapon you can have for a suit is having it properly tailored to fit you. Stores like Men's Warehouse or Jos A Banks, while affordable, have crappy tailors. Definitely do not skimp on the tailoring. Also, make sure the suit isn't too wrinkled. When traveling, fold it up. Do not wear the suit jacket while traveling if you can avoid it. If you can, the night before, hang up the suit in your bathroom while you shower to let it steam a bit.
Here is a great youtube video on folding your suit:
If the suit is tailored properly, the shoulders and chest area should be snug. The abdomen area should taper to a "V" shape. The sleeves should come just to the wrist. The pants should not be baggy and the ends of the legs should JUST cover the shoes.
Shirt: Non iron, slim fit shirts are the best. I LOVE Brooks Brothers for this. I highly suggest light blue or white for dressing noobs, they are the most versatile with almost any suit. A solid pattern is also the EASIEST to match with a tie. You can't go wrong with a nice solid, light colored shirt. That kind of shirt will allow you to pick out a strong matching tie easily. Other colors to consider, light purple, pink, green (for gray suits), yellow (for brown suits). Also, make sure you IRON that shirt the night before; UNLESS it is a non-iron shirt. Do not iron non-iron shirts. You will ruin that thing that makes it a non-iron shirt. Those are meant to be hang dried.
Tie: I hate ties with gimicky patterns them (i.e. chili peppers, whales, horses etc). The pattern should be simple and non comical!
The biggest error I see is
color. Here is a VERY simple rule to follow: a color on the tie should match with the color of your shirt. Generally the lightest color of your tie is the easiest to choose from. This will allow a good contrast of the darker colors to stand out.
Pattern: Don't try anything too crazy. This is why I recommend a solid shirt. The pattern of the shirt should not be exactly the same as the pattern of the tie. i.e. the stripes shouldn't be the same size, nor should they be going the same direction. Solid ties and solid shirts kind of scream formal dinner parties but it may work.
Belt: Just follow the age old rule- the belt color should match the shoe color.
Shoes: The bane of my existence when I was researching this stuff. I still have a hard time explaining it and it is more of a "when I see it, I'll know if it works" sort of situation. Here is a very simple guide:
http://imgur.com/GYOyA
Pocket Square: Lose it unless you are 100% confident about wearing one.
Tie clip: I always wore a tie chain. I felt like this set me apart very subtlety from other applications. Very few people wore this. I always joked that it was my "bling."
Though it seems daunting, it's not that hard to dress as a man (I have no comments for women, sorry!). I am no dressing guru but a little bit of research and I was able to learn at least the basic rules. I was even able to take the ideas into my regular day dressing and start dressing better in real life.
Look at this:
http://imgur.com/gallery/tu7yM
Read some askmen.com or GQ.
Good luck!
PS: Dinner attire: no one really cares as long as you are covered and don't smell.