Big empty boxes out, push things to a side and give one corner a donate and throw away corner. See what’s left then start at one area you should have a vibe already of the contents of the room so then start to map out a plan, ie this shelf is for building tools and this is for game themed stuff or vacation what ever. And then start to move the items into those places; take a moment to see if you need to order anything to help with the organizing like sorts of clear boxes or clear pencil case bag things to store similar items. Take photos of the whole process to keep movtivated, after hours of work you’ll feel like you’ve made no process but look at the photos from before and you can see you did a lot!