How do I know what kind of foundation I should buy - a liquid, stick, cream, powder or mousse?

It really depends on the condition of your skin and what kind of coverage your want.
The advantages of a liquid foundation?

A liquid foundation tends to blend easily and offers a wide range of coverage from sheer to full coverage. If you have oily skin remember to use an oil free liquid and an illuminating one if you have dull or dry skin. If you need to cover breakout you should use a highly pigmented foundation.
The advantages of a stick foundation?

Stick foundation is excellent for spot coverage on the chin or around your nose
The advantages of a cream foundation?

If you are looking for more coverage and a longer lasting foundation use a cream foundation. You can keep if from looking too heavy by using a foundation brush to apply it which will spread it lightly.
The advantages of powder and mousse foundations? Both will give you lighter coverage and look best on clear and evenly pigmented skin.
How do I find the perfect shade of foundation?

Go to the department store and try a few swipes of different shades on your jaw line and go outside with a pocket mirror and see which one blends seamlessly into your skin. link....