This is an especially helpful tip if you're buying a suit somewhere that doesn't have a knowledgeable salesperson on staff.
Your suit pant legs need to be the correct length - not too short, and not too long. If they're too short, it will make you look ridiculous; if they're too long, it will make you look like a little kid in a grown-up's clothes.
The trick here is to make sure that your socks aren't visible when you're walking, while at the same time avoiding stepping on the bottom of your pants. You may need to take someone with you to make sure that the length is correct.
The easiest way to make sure you have the right length is to try on the pants WHILE wearing the shoes that go with the suit.
Make sure that when you're standing that the bottom of the pant leg is about a 1/2 inch above where the top of the heel. Trust me, this makes all the difference in the world.