How do we know which shampoo is best for us? how to choose the right one?

Shampoo: Most common shampoos are effective and tolerated by most people. Occasionally people will have an allergy to a fragrance or preservative, but that is not common. If your hair is dry, use a shampoo + conditioner. If oily, use one for oily hair (cleans away oil better, but may be more drying). Try a few brands to start--most will likely work for you.