When dealing with an overwhelming mess, sometimes the first step is even putting a bin bag in the space, plus a box/bag for donations and another box or bag for taking things to other places to put away. Do this in each space there's a mess. Then there's no barrier when you go back to a space and look for a few things that need to be thrown away. You'll be able to spot some things that are easy to get rid of. You can then set a timer and (if you're me) put music on and get started tackling it in little time slots. You'll probably find you'll do more than you expected as long as you don't have to leave the room in that time slot! Having to go off to get a bin bag etc means you lose time and might get distracted! Having those things set out in each problem area can make so much difference to getting stuck in.
Once you're going, I also (as others have) recommend The Organised Mum Method. It certainly helped me develop a routine that suited me and helped the house feel so much better. I have read flylady too and whilst I prefer TOMM myself, if I have a week where my usual routine isn't possible, I will split the tasks and do it that way if I need to. In my house, with the people I live with, TOMM makes the house seem cleaner than the flylady way, as with TOMM somewhere always looks clean and with the flylady system in my house, it doesn't seem to notice so much - less impact on the chaos in one go. I can completely see how it could work in a different home though.
Deal with the decluttering separately from the introduction of daily routines, then you'll be getting on top of things rather than making one area better while others get worse. If you always have clean sinks and clean toilets in the house, for example, and you always vacuum SOMEWHERE, you'll get it done.
Personally, I found simplifying my use of products made a difference too. I use a wet multi-surface cleaner or I use Koh. For my main cleans, I use Koh as you use that with a dry microfiber cloth. I prefer that as I find it quicker, but keep multi-surface cleaner in the kitchen and bathroom for cleaning quickly through the day/as needed and for others to use. My husband likes Method bathroom cleaner, so we have that in the bathroom too and he uses it to clean round after his shower. It means it gets cleaned an extra time in the day, so I am happy with that!
In a zone (I have divided my house into 4 zones), I bag up things that need to go elsewhere (I live with untidy people) and then I dust from top to bottom with a feather-type duster (initially, I used a vacuum to dust, as I have asthma, but this is faster and the duster I use holds onto the dust as long as it's only been a week or less since the last time) and then I use koh to clean any glass and windows. This takes less than 5 minutes in most rooms now I have got going. Even cleaning large patio doors only takes 2 mins. Probably less than 1 now. Then I will vacuum and if needed, use my Koh mop to quickly do the floor, if relevant.
Bathroom: I clean the bath/shower, sink and toilet after each shower, while my body lotion soaks in, using a multi-surface cleaner, as I do that while the surfaces are wet. Once a week, I do a Koh clean later in the day, when it's drier. I spray everything, then wipe the fixtures and sink with a microfiber cloth quickly, before using the koh mop to wipe down the tiled parts of the walls and then the floor. Whole thing takes a couple of minutes and using the mop saves me ages! Doing this makes a difference to how clean the bathroom feels over the rest of the week.
I leave multi-surface cleaner (I like Ecover) in a spray bottle in the kitchen because that's easier for other people in my family to use because they don't understand that with Koh you only need a tiny bit! They would waste tonnes and still probably not clean up half the mess. Also - washing things down with any kind of soapy water is basically all you need to do. It doesn't have to be fancy!
I love 2 things in particular from TOMM and FL. I think it's from TOMM where she says about being your own future friend. I think it's FL who said about making sure you have a clean sink makes a difference. Sorry to both if I got that the wrong way round. Also that saying about 'Don't put things DOWN, put them AWAY' - all 3 of these things have been game-changers for me! You will find your own way and find what works for you.
I seem to have got carried away and realise I have said loads and could carry on! I will have repeated what others said too, I expect. These are all things I wish someone had said to me when I was finding things tough. My children used to help when they were younger, but now they are adult it is not what happens. Yes I agree it should. Different battle. This way, it gets done and I feel better. I do it for me. I don't put their stuff away. I put it all in a shopping bag and put it in their room/s and other than that, I stay out of their rooms. No way I am cleaning in there. I move their stuff in a quick way, so I can clean. Less stressful for me, cleaner house and they can moan all they like if they didn't want things to be put in that bag in their room. They should have sorted it out themselves.
You will be so pleased once you get started though! If you start by taking out some rubbish you can see, you will notice a difference quickly. Good luck!