The first thing you should do when you're shopping for pants, is find a pair that fits. As always, they don't have to be expensive, as long as they fit well. Which means you might have to visit a tailor. They shouldn't be too low rise, and if you can't find a length that's good for you, err long and get them hemmed. Especially for work, these shouldn't be too skinny, you shouldn't feel like they're about to rip off of you; baggy pants are also a crime, your legs shouldn't have too much room to move around inside the pant leg. And no pleats. Ever. Straight leg is going to be your best bet, because even those should taper about an inch from your thigh to your ankle. Even if you're a set size, every store's pants will fit a little differently, so find your magic pair...then buy all 6 necessary colors in that style (plus more, if you wish!).
And so for the six. In your closet, you should have the following: black (non-suit) pants, gray pants (light or dark, but both is preferable. You can do dress pants or khakis, depending on which your life demands more), darker khakis, lighter (but not too light) khakis, green (hunter, not kelly) and navy. Owning these will insure a wide variety of shirt/pants combinations, and no matter which pants you choose to wear, you will always look better than casual. These will span a range of different dress codes and you won't always look like your mom laid your clothes out for you to go to a school dance. This mix will give you neutral staples as well as updates to those so you can vary what you wear and easily give your work or nighttime attire a more stylish edge.