Starting to declutter is really hard.
I honestly only got on top of it because I was working my 3 month notice, from home, in a role where I'd pretty much already handed over my job to my replacement! So I spent 3 months decluttering, whilst I kept an eye on my emails!
I started with easy wins like books, cookery books, dvds (we didn't even own a did player!), cds.
The key is to sort out a load and drive it immediately to the charity shop or tip. Don't stack it somewhere to deal with later, or you'll create another job for yourself. Another tip is - try not to worry about money that has already been spent. Yes, it's gutting to get rid of £200 of cookery books, but does keeping them mean you've spent any less? No. The money's gone, and the books are just a burden, so get rid.
Anything you've not used for a year or so, or find and forgot you owned - get rid.
Don't worry too much about sentimentality. I have a handful of items that I've moved with me from house to house and which I'll never part with but, for everything else, remember "Things aren't memories". So that teapot that your mum bought you, that you'll never use? Get rid.
With clothing, get rid of anything tatty, which doesn't fit, is stained etc. Then ask yourself "Do I ever reach for this?" We all have items of clothing that are in good shape, fit you well and so on, but we NEVER choose to put them on. So get rid.
We only had a 2 bed terraced house, and it always looked tidy, but I spent that initial 3 months decluttering, and then chipped away for several further YEARS. It's incredible how much stuff accumulates and is tucked away in every corner of your home.
We now have twice the space (we moved) but a 1/3 of the stuff. Cleaning is so easy and everything has a place. I try not to buy too much new stuff, and have a 1-in-1-out policy, but constantly declutter to make sure its never an issue again in future.
My inlaws house breaks my heart. They're borderline horders - the house is piled high - and believe everything they own is either useful, valuable or has enormous sentimental value. All that's going to happen is, they'll die, us kids will hire a skip, and bin the lot.