Prepare your brain? Like, hit it with something? Or maybe eat tons of sushi so you can learn japanese with less effort? No, nothing like that.
To learn japanese you must really start with basic grammar: the order of words, particles and some basic verb forms. You will learn some basic vocabulary on the track, but what comes next is for you to decide. Either you go with deep study of vocablulary and grammar, or you can watch like tons of anime and japanese video footage to get the hang of some spoken language, and if you're smart enough, get some vocabulary and even grammar knowledge from it.