Little Black Dress.

These pieces can be tweaked to match your personal style, but there's one thing they should all have in common: A perfect fit.
You will need a little black dress, plain dark jeans, a crisp, white button-down shirt, black dress pants, white T-shirts, a black skirt, a trench coat, professional laundering (optional)

Steps
STEP 1 Invest in a little black dress
Invest in what fashionistas call the LBD, or Little Black Dress: A simple, elegant sheath.
Try a sheath dress in navy, brown, charcoal, or red, too. Just make sure it's a solid color.

STEP 2 Find the perfect jeans
Have a go-to pair of jeans that fit well and can be dressed up or down. That means no rips,studs, or fading – just a dark pair of plain jeans.

STEP 3 Get a crisp white shirt
Own at least one crisp, white, button-down shirt. Look for fitted ones that won't bunch around your hips when you tuck them in – but won't tug at the buttons, either.
Unless you can afford professional laundering, avoid 100 percent cotton in favor of a no-iron fabric.

STEP 4 Have black dress pants
Buy at least one pair of black, all-season pants in a basic cut that works just as well with a sweater or jacket as it does with a shirt or blouse.

STEP 5 Stock up on white tees
Stock up on white T-shirts – crew neck and V-neck, long-sleeved and short-sleeved.
STEP 6 Get a good skirt
Get a good black skirt. A knee-length, A-line skirt is universally flattering. But if you have the figure for a pencil skirt,go for it.

STEP 7 Buy an all-weather trench coat
Buy a classic, all-weather trench coat. Don't skimp on it; a well-made one will last you a decade or more.
Sixty-six percent of women own at least one "little black dress."