writing a book

I think i am going to write a book, though im not 100% sure what to write about or what to put in it, maybe if i just do a lot of writing an then just put it all in order  then see whats happens, tough its so hard with what to write, as other people will be reading it, i dont want to do an autobiography though i dont want to have a fiction book either Im sure i'll figure something out, just gotta start putting stuff down,