We recommend you start by checking out all the different bathroom styles that are out there - see this complete guide to popular bathroom styles. Once you have an idea about the type of style you like, it's time to get practical.
Start by measuring your walls and create an online floor plan. Then place key fixtures, particularly those that are "fixed" e.g. for plumbing purposes. Then create copies of this plan where you can play around with furniture like cupboards and dressers, and accessories like mirrors and plants.