What causes spots exactly is not know. Dermatologist suspect that fried foods, greasy foods, unneccesary sugar intake, etc cause spots. It's also genetic. If your parents have good skin, chances are, so will you. If not, then you're in for a rough time. But even the most beautiful, flawless skins get spots sometimes.

To keep your skin as nice as possible, try to follow these tips:
Drink lots of water!
Eat healthy foods
Don't smoke
Don't drink (much!)
Avoid caffeine
Avoid fried and greasy foods
Cleanse and exfoliate your skin daily
Clean off your makeup every night
Use special masks for your skin weekly
Avoid foundation, blush and concealor if your skin is really bad.
Concealor can look a bit obvious, so only use it for a special occasion. Plus everyone gets spots, why be embarrassed??!

If your skin is really bad, see a dermatoligist or your GP who can refer you to a specialist.