Start wherever you can even if it is the junk drawer. You will feel a larger than life shift from one small clearing and might even find your spare key!
Decide which areas in your home bother you the most and start there.
To get real momentum, carve out at least one hour 3 days each week. Each time you start clearing make sure you have at least one solid hour to work so you can take advantage of the momentum you will create.
Always leave time at the end of each clearing session for putting donations into your car to haul away the next trip out!