Log in to your Amazon account and get to where you can manage your titles/purchases (Your Kindle Library). Each book will have an 'Actions' button to the left of it. If 'Loan this title' appears as one of the options. You can loan that title. All you have to do is enter the email address of the person you want to loan it to. Once they accept the loan, it will be unavailable to you for two weeks (and they have two weeks to finish it as well). At the end of that time, it will become available to you again, but you won't be able to loan it again. One loan per book.
Or do what Harry said.
