i am writing a book and i have used some of other peoples work how do i do this legally?

If you have only paraphrased their work, you can write a bibliography or reference list to show that you have used their work. If you are copying/have copied more than a three to four sentences, don't! That's plagiarism and you could get in big trouble for it. Instead, paraphrase, and compile a reference list.

Edit: it doesn't matter what genre the book is. Plagiarism applies to any sort of media (books, electronic media, radio, speeches, etc.)

If you actually quote them, like this:

Tom Smith says "many people have asked my secret, I have no secret, it's just hard work"

You have to ask permission of the author and his publisher and may have to pay a fee (sometimes this is waived).

Look at the front of most fiction books and you'll see \"The excerpt from Tom Smith's "Stories of Life" is published by permission of Chancery Press, New York"

Unless they are co-authors, or unless you plan to legitimately cite very short quotes from other works, there is no way to legally do this.