It's hiring season, so lots of interviews and people starting new jobs. Plus, some of you just need the fashion advice. For reals. Let me preface by saying that if you've got your own style and flair, this isn't for you. But if you're the kind of dude who is clueless about that sort of thing (most guys, I imagine), hopefully this will help.
Basic advice-
Smaller is better. There was a time when big, baggy clothes were in style. It was a long time ago. The XL sweater and wide-leg khaki look is generally considered out of style at best, or juvenille at worst. You need to buy clothes that fit. Most stores have websites with fit guides, like this one for Gap/Banana Republic
Apparel, Handbags, Shoes and Accessories for Women and Men | Banana Republic. Notice that the sizes correspond to measurements (chest, waist, neck, etc). Measure yourself, or even better get help at a department store like Macy's or Nordstrom. You'd be amazed how much better you look with clothes that are the right size for you.
Throw away your tube socks. DO NOT, EVER, wear white, athletic socks with anything other than athletic shoes. The color of your socks should go with the color of your pants. If you're wearing khaki pants, get khaki socks. If you're wearing black pants, always wear black socks. If you're wearing grey pants, you can pull off black or grey socks. Don't clash. And again, don't wear gym socks with dress shoes.
Your tie can make you look great, or make you look terrible. The bottom of your tie should line up with the middle of your belt buckle. No exceptions. Modern ties are slightly more narrow than ties from say, five years ago. But the difference is subtle, and probably not noticeable to most. When choosing a color or pattern, think about what shirts you will wear it with. If you are going for a conservative look, get yourself some solid color white and blue shirts, and get some red or blue ties, not necessarily solid color but nothing too fancy. If you want to dress it up a bit, go for stripes or patterns, but try to make the ties and shirts have at least one color that matches. For example, if the shirt is blue and white pinstripes, look for a tie that has at least some blue color similar to the shirt. Getting good at matching ties to shirts takes practice. I recommend you start with the basics, then experiment as you get more comfortable.
Belt + shoes = match. Brown shoes for when you're wearing a brown belt. Black shoes for when you're wearing a black belt. No exceptions. If your dress shoes aren't brown or black, you're doing it wrong.
Steal someone's look. Sounds gay, but it isn't. Find a guy who looks sharp, and see how he pulls it off. It helps if the guy is a similar build, skin tone, size as you. Don't end up wearing the same thing as him every day, but don't be afraid to copy what looks good. Women do it all the time.
Take a (nice) girl/boy shopping with you. This is important. Don't take that girl/boy you call at 2am when you're drunk with you. Don't take that girl/boy who has never had a job in her life. Don't take a girl/boy who is five years younger than you. But do take a woman/man who does pay attention to what respectable men wear. Maybe she/he has a job at a law firm, or she's/he's getting her/his degree in business. If she/he thinks you look good in your new wardrobe, other nice girls/boys will, too. This is a good thing.
Advanced advice-
No wrinkles. Lots of places make wrinkle-free shirts these days. They are a godsend. Do not be wrinkled shirt guy. Either get wrinkle-free, learn to iron, or shell out money for dry cleaning. But be aware that repeated dry-cleaning will ruin your shirts over time. Just make sure you don't spend $80 on a new dress shirt, only to have it look like you just pulled it out of your hamper.
Consider having your clothes tailored. A lot of guys have trouble finding clothes that fit off the rack. I'm tall and thin, so mediums tend to be too short, whereas larges tend to be gigantic. I often buy large shirts and have them altered to fit me. It can be a bit of a hassle, but they look great. Find a dry cleaners that does alterations (most do), and bring them a shirt you want to fit you better. The most common thing to have altered is the girth- American dress shirts are made for fat dudes, so they have a ton of fabric around the middle. If you get this altered, it will fit more smoothly and will stay tucked in. It will also prevent your shirt from getting all baggy around the waist, which makes you look like a kid wearing his dad's dress clothes.
It's better to be overdressed than underdressed. These are words to live by. Consider the consequences. Yes, you probably look a bit stiff if you show up wearing a suit when everyone else is jeans. But you look like a clown if you're wearing jeans and everyone else is in a suit.
Personal tip-
Boxer briefs. Your novelty boxers were cute when you were 17. They're ridiculous at 25. Sleep in whatever you want. But when he/she invites you back to his/her place, do you really want to be wearing funny underpants?