Where to start... by: Samantha w/ Help Organize Life
The first place to start is the place that bothers you the most. Even if it is in the corner of the room it is a place to start. I would also employ help from your family to help start. You can make it a game for the kids or reward them if they can come up with the most creative way to store their toys and to keep it clean. I have a 4 year old myself so I know how a perfectly organized room turns into a bomb shelter within a day. The main thing is every night before bed make it routine to go into your children's rooms and have them help you pick up and put away their toys. Also, if they have a lot of toys I usually wait until my little girl is at preschool and I go through all of her toys and pick out the ones that she has not looked at or played with in a long time and weed out old and broken toys. I either donate or throw away before she gets home and she is none the wiser. This is a way to keep on top of the toys.
As far as your office goes, put all paperwork in a file box. Then when you have down time go through a box at a time and make categories that make sense to you with file folders and file the paperwork. Get anything that does not have anything to do with your home office out of there. This might be the first place to start because you will see instant results that will motivate you to keep going. Also, figure out what function does your office need to have. What things do you need to accomplish? Answers to these questions will help you design an office that works and help you weed out what is not working in there.
The best advice is to start small. Set aside 30 minutes to declutter a room or a small space and the rest will be done before you know. And if you truly don't have the time look into hiring a professional organizer who can get it back into a manageable state where you can start fresh.