Separate types of toys into storage boxes (use nappy boxes if need be) then put a label on it and put away in a cupboard. Before one box comes out the old one has to go away. I find the kids play with things more if its not accessible all the time!
It can be quite overwhelming. I’d write a list of which rooms you’d like to start with, giving each room a week to get through it (or whatever pace you want). It just helps to break it up into smaller work loads so you don’t get overwhelmed.

I get my kids to help de-clutter their things – I ask them to find 5 things they would like to give to a child less fortunate than them and they are usually more than willing to find something.
If they are naughty they loose a toy to a box, if we don’t miss that toy after two weeks, it goes.
Clear out clothes regularly – sell them or give them to charity or a friend.
Get a filing cabinet and store paperwork etc in it. I also store sports stuff in there and arts/craft.
Invest in storage. Whether it is fixed or removable it all helps.

live by the rule:
If its not useful or beautiful then throw it out!!

Ive just had to do this with my 4 year old his bedroom is so full of toys its unreal, so i sat down and threw out anything that i hadnt seen him play with in the last 6 months i was actually surprised with the amount he doesnt play with anymore and i think the only reason for that is he has far to much to start with.
Ive given four boxes full of toys to the womans and childrens hospital, and my sons room is now back to some sort of manageable mess lol

One way is to stop bringing new stuff in – ask “do I really need it?” before you buy it. Okay, I know this isn’t easy with kids, but it will start cutting some of the new clutter. That was an important step for me.

With the old stuff, ask yourself how often you use it. Or, try putting it in a box, and if you haven’t missed it in a month, get rid of it. I find this really hard too.

And there are some things you just have to hang on to till it’s the right time. I’ve found it really hard to part with baby clothes (though we won’t have another), but a friend having twins in December has been the incentive I need to pack them up and give them away.