The Kids Closet
If you have more than 1 kid, just start in one room. If you don't have any kids, you get to pick a room of your choice - maybe the guest room or a home office.
The key for this room is to make this challenge for a limited amount of time. You could easily spend all day working in here, but that's not the goal for now. Give yourself 5 or 15 or 50 minutes... whatever fits in your day. Pick off the "low hanging fruit" - the things you can see immediately do not belong, no longer fit, you don't like, etc.
If your child is old enough, get them involved in the process. Let them know they can give their unwanted items to kids who may not have something like it.