Start with one thing you already know you don't want. Not a room, not a plan. One item, out of the house today.
On iPhone and iPad, that's how Declutter Quest starts too. The first time you open it, the home screen asks you to actually let go of a real item, then tap the button to log it. No account, no setup.
It sounds too small to matter. It isn't. The hard part of decluttering is rarely the sorting. It's the first decision, plus the feeling that you have to do everything before anything counts. One logged item gives you a number that isn't zero, and a number that isn't zero is surprisingly easy to add to.
Some easy first items: the pen that doesn't write, the charger for a phone you sold, the menu from a takeout place that closed two years ago.
If you want more structure, the Challenges tab has a 10-Minute Sprint and the 30-day Minimalist Game. They can wait until tomorrow. Today, one thing.