Where to begin

Right so i am going to try to write a book and see where it takes me...it is something i have always dreamt of doing and have decided to take a cue from my brother and just go for it...i guess the worst that can happen is that i don't get anywhere with it and just abandon the whole idea. So now where to begin...i've always heard to write what you know so i guess that is where i will start...stay tuned.