The most important word to remember when you're thinking about businesswear is pieces. Versatile pieces are definitely a key concept in every wardrobe, but for businesswear, they become even more necessary. A few key high-quality items of clothing will be able to provide you with a fine basis for your working wardrobe.
Everyone who works in an office should own at least one buttondown shirt. That's just the law (seriously). I would probably have at least one white one, and a few others of appropriately muted colors and patterns. Blouses are great, as long as they aren't too revealing. Basically, in an office setting, the general rule is to attract as little attention to yourself as possible based on your clothes. You obviously want to draw everyone in with your brilliant ideas, stunning work ethic, and exceptional personal magnetism--you just need to look professional while doing it. My very subjective and easily contradicted guideline is this: if I would wear it on a date, I probably shouldn't wear it to work.
This isn't to say you can't have some fun or personality in your look. A great way to do this is with blazers. I love blazers. They are italicized forever in my heart. There are blazers (okay, I'll stop) in a variety of colors and styles; for the most part, you want one with darting on it, to give you some structure and keep you from looking too boxy.
One of the bonuses of blazers is that they allow you a little more fun with your under-blazer choices; a jewel-toned shirt might seem like too much on its own, but layered under a dark blazer, it's a sassy pop of color! Another great way to personalize your look is through the cunning use of vests. For the office in particular, I especially recommend sweatervests. Actually, I recommend sweatervests for pretty much anything. I'm wearing one right now!
They add an air of saucy nerdiness to an outfit, while still remaining very practical and appropriate. In more wintery times, a nice sweater (not one with a Santa Claus or any kind of puffball on it) can keep you warm during your commute and stylish in the office.
In my prior wardrobe essentials post, I showed a Polyvore set with some of my favorite pencil skirts. The pencil skirt is still the choice for office style in my book. A nice A-line is also acceptable, though stay away from anything too flouncy (as difficult as that is--we love the flounce!).
For work, pants should be black, brown, gray, navy, or some subtle pattern in those shades. I'm personally a huge fan of a faint windowpane pattern, which can add a kicky retro flair to the outfit.
As far as style is concerned, tight is obviously out. The office isn't the place to rock a pair of skintight anything, no matter how awesome they are. I'm personally a fan of wide-legged trousers for more formal situations, but whatever your preferred non-jeans pants are will be fine, as long as they fit properly. Ideally, you'd be able to go to a tailor to get them fitted perfectly to you, but hey, most of the time it's exciting to have enough money to get pants, let alone tailoring. For this reason, I usually choose to buy up instead of down. Pants that are a little too long can be counterbalanced with a heel, or hemmed, and if they're a bit loose in the waist, they can be held up with a belt. Never ever buy pants that are too short for you--they look ridiculous and honestly, if your shoes are having a party, they should invite your pants to come on down. You don't want to have rude shoes, do you?
Shoes can be a great way to show your individuality in the office, but you can't go overboard. It's fine to add a subtle pop of color (subtle is one of the buzzwords for office attire, if you hadn't noticed), but work isn't really the place for your hot pink stilettos or Ugg boots or anything made with lucite. Consider what you do all day: if you're going to be making copies, delivering mail, making calls, or on your feet for any appreciable amount of time, you'll probably want to pick lower heels. Sky-high heels and platforms are not appropriate for the office.
Less is more when it comes to office accessorizing. You still want to be fabulous, of course, but the place for your chunky costume jewelry is not your desk. A cute brooch is an easy way to add a little bit of fun to an outfit without looking unprofessional or "too much."
For the most part, it's always better to err on the side of "too little" when it comes to the office. I'm going to render everything that I've told you in this post useless and say: it really depends. Each office is going to have a different dress code and different standards of professionalism. The best way to know what's appropriate to wear is to look around and see what your coworkers are wearing (look to those who have been with the company longer; they'll have a better idea of what will fly and what won't). At one office, dangly earrings and a huge necklace would be fine, where at another, anything beyond gold studs is considered incredibly gauche. The same goes for pretty much any part of your office wardrobe.
So my real advice is this: take cues from coworkers to figure out what's appropriate (or ask your supervisor!), but the suggestions I've made here should help you play it safe in just about any office. I've tried to provide a general guide of pieces you should look for and what sorts of things you can build a professional wardrobe on. Any questions, class? (Hope this helped, bb!)