All above are excellent suggestions.
One important thing to know is math... A 6ft tub will not fit into a 5 ft space,,,,
Electrical and plumbing- qualified tradesmen make sure there are inspections, be there for inspections too if you can. Ask questions.
Do research on contracting, contractors, how electrical works (from power panel to end usage), how plumbing works (from supply to drainage). Best place is internet. Utube to see how things are done. Home depot/other home centers to see materials and to ask questions of people in plumbing, electrical sections.
know who is responsible for what.
If you are new to contracting and have several jobs, do it in stages, contract 1st job (bathroom), then contract 2nd job (kitchen). If you have a general contractor, ask for his tradesmen qualifications or who he subcontracts to. Sometime these change. If your have a contractor and he is working on your place, make sure there is a permit raised.
Phone up the local city government office that does planing/issues permits ask about inspectors etc.
If you have to open up wall/floors, be prepared for extras... just in case.
go to local library and look up books on contracting, building, etc.
The main issue you will have to deal with is organization. Your time, the order of specific timings for things to be done (schedule of events) and access for the contractor. I would suggest, is to be their yourself or have someone you trust there all all times....
During the work, take before, and after pictures. If a wall is opened up take a pic, you never know when you need to know exactly where that drain pipe is going through that wall etc.