The most common question words in English are the following:

WHO

WHO is only used when referring to people. (= I want to know the person)
 

Examples:
Who is the best football player in the world?
Who are your best friends?

 

WHERE

WHERE is used when referring to a place or location. (= I want to know the place)
 

Examples:
Where is the library?
Where do you live?
 

WHEN

WHEN is used to refer to a time or an occasion. (= I want to know the time)
 

Examples:
When do the shops open?
When is his birthday?
 

WHY

WHY is used to obtain an explanation or a reason. (= I want to know the reason)
 

Examples:
Why do we need a nanny?
Why are they always late?

Normally the response begins with "Because..."

 

WHAT

WHAT is used to refer to specific information. (= I want to know the thing)
 

Examples:
What is your name?
What is her favourite colour?
 

WHICH

WHICH is used when a choice needs to be made. (= I want to know the thing between alternatives)
 

Examples:
Which drink did you order – the rum or the beer?
Which day do you prefer for a meeting – today or tomorrow?
 

HOW

HOW is used to describe the manner that something is done. (= I want to know the way)
 

Examples:
How do you cook paella?
How does he know the answer?
 

With HOW there are a number of other expressions that are used in questions:

How much – refers to a quantity or a price (uncountable nouns)
 

Examples:
How much time do you have to finish the test?
How much is the jacket on display in the window?
 

How many – refers to a quantity (countable nouns)
 

Examples:
How many days are there in April?
How many people live in this city?
 

How often – refers to frequency
 

Examples:
How often do you visit your grandmother?
How often does she study?
 

How far – refers to distance
 

Examples:
How far is the university from your house?
How far is the bus stop from here?