To start with you probably want to have a centre part and nice clean hair. If you must have a different part then you can change this later but really because the braid goes across the top of the head the exact part becomes less important. Next step is to make a circular part all the way around the head. The hair left should be a thin section around the outside of the head. The hair in the middle can be tied back and then put into a quick bun to get it out the way:
this video. It definitely worth a watch if your having trouble parting your hair right or need more help on how to do the braid.