Start by going through each room, and making a list of all the things you do in that room. Next decide the functions of the room and how you could make it simpler and function better. Even if you can only do this in one or two rooms it will reduce your stress when you are in those spaces.
Create creative systems for doing your chores, meal planning, shopping, and dealing with everyday necessities. The easier it is to do those things you have to do everyday the calmer your life will be.
Simplifying is a great way to making your life less stressful, relaxing, and peaceful.