A simple look is always a look that won't have people staring, always a look that won't have people talking about you. You can feel confident looking simple.
1. Know that after showering at night, part your hair to the side. If you have bangs, part your hair in the middle to keep them down and straight. However you may part it, make sure that your hair will stay out of your face.
2. Get dressed wearing a pair of classic blue jeans. Classic jeans are not very skinny and tight, they also do not flair out a lot and are not baggy. They fit you just right and should come down just above the balls on the sides of your feet. Then, wear a solid color t-shirt. The length of the shirt should just cover wear you put a belt on your jeans. Wear simple sneakers like K-Swiss.
3. Wear your hair in a ponytail smack dab in the middle of the back of your head. No bumps. If you have bangs, comb the hair from your bangs back in the ponytail. If you parted your hair to the side, don't flip your hair back in the ponytail. Just leave it neatly parted.
4. Wear some clear lip gloss, maybe some mascara.

For the real simple look, don't paint your nails bright colors. A simple French manicure on short nails looks fine.
Be yourself. The simple look doesn't make people who like to stand out feel very confident as people who like just blending in.
If you wear your hair down, it will look more simple if all of your hair is the same length and has no layers or bangs.
If you change from standing out to being simple overnight, people might talk and stare.