How to transfer my Kindle to somelse?

You need to go into the settings and unregister it from your account, then all they need to do is to create an Amazon account if they haven't got one, then go into the same settings and register it to them. Any books that you bought through Amazon won't be available to them, books you put on it with third party software like Calibre will.

You can also factory reset it if you prefer, that both wipes the memory and unregisters it at the same time, putting it back to the way it was when you first got it out of the box. Then, as before they need to register it to their own Amazon account.