How do you start a book? give a descriptive answer.

Depends on the book you're writing. Narrative, informative, or argumentative? Though, no matter what type of book you're writing, you should always have a well written hook that would capture the audience's attention. 
For example;
If you were to write a book about tornadoes, you could start it off with,

"Boom! Crash! It's pitch black outside. You try to find your way to safety until you hear sirens going off. Heavy winds are pushing you from side to side. Where should you be going? What should you do? It seems like you're experiencing what is known as a tornado in your area."

^With a capturing hook like that, the reader becomes interested in knowing more about your topic, and reading on to know how to deal with a situation like that in the future. Once you've grabbed your audience's attention, they are now ready to listen what you have to say about your topic. 

Step 1: Pick a Genre, this will make you understand how to write
Step 2: Start from the End, this lets you know the ending, so now you just want to build up the story to end like that.
Step 3: Create Your Characters, give personality traits, let the reader meet them.
Step 4: Make an Outline
Step 5: Write the First Draft, this one has many mistakes which you fix in the final draft
Step 6: Get Yourself a Drink, you've earned it at this point :D
Step 7: Rewrite, make this one perfecto'

Try to give a giant hook in the beginning and get someone to read it when your done before publishing :)