Start with what does not work in the current bathroom. Is the shower cramped? Is there too little storage? Would moving the toilet make better use of the space? A useful brief explains the problem, not just the style you like.
A complete renovation may involve removal, pipework changes, wall preparation and several finishing trades. Ask for a quote that sets out which tasks are included, who carries them out and what you need to supply.
Existing conditions can affect the plan once fittings are removed. Discuss how any extra work would be explained and agreed before it goes ahead.