Way before you ever wield a sledgehammer, the best place to start your kitchen remodeling project is with a thorough, clear-eyed assessment of your kitchen's current state and what features and functions you require in a remodel.
Kitchen design is a very specific skill, so you'd also be wise to consult with a designer certified by the National Kitchen and Bath Association, who will have the know-how to create solutions that fit your budget as well as your lifestyle (and that includes keeping the actual remodel moving along so that you can get back to actually using your kitchen as soon as possible).
From there, what's tackled first will depend on the extent of the remodel to which you've committed. Just take care not to create extra problems and expense with such whoppers as moving plumbing around and going into walls. And if your kitchen has "good bones," capitalize on them with smart updates that make the most of your kitchen remodeling dollars.