I would start with something easy, clothes! Most people keep clothing in good enough shape to be donated but if you have some items in great condition, you could sell them on eBay or to a thrifting store. Otherwise, donating them is a great option.
Once you have gone through clothing and similar (shoes, purses, etc.) then focus on one room at a time. Go for the kitchen, then bathroom, then closets, and so on.
I think going with one room at a time is best, just like VINAMI said. This also works for moving. It keeps you organized and if you need to stop for the day, makes it easier.
I tend to start in drawers and closets, cupboards, things like that. That is where most of the clutter ends up. I sort everything in boxes - keep, sell, donate, bin. I do this every spring!
Just think about the things you have too much of. Like do you find yourself having too many handbags, shoes, clothing, etc? then start there
