OK, you are not going to be able to keep the place clean/tidy without doing a big blitz first!
First buy some big strong bin bags - you are going to be ruthless and throw things away!. And some nice, stackable plastic storage boxes (try IKEA, B&Q or homebase). And some box files and clear plastic folders for paperwork. Lots of big sticky labels, post it notes and a big black marker pen.
Set aside a chunk of time (at least 2 hours), and pick a room.
Now, start with everything that is sitting out on a surface. If it is paperwork start organising it. Start with who it belongs to - you/DH/DCs etc. Every piece of paper needs to go into a clear plastic folder depending on what it is related to. So you will end up with folders like "My Car", "DH Car", "My bank stuff" etc. Don't be too micro organised at this point - just get it into folders/box files.
Make sure you label all the files as you go along (I find a post it note stuck to the inside of the file makes a good label - you can see it from the outside but it doesn't fall off and isn't permanent).
Throw away any pieces of paper you don't need to keep. Cuttings of recipes you are never going to cook? Bin. School news letters? Bin. Bank statements from more than a year ago? Bin (you can always get duplicates from the bank in an emergency). Throw away all the old magazines you are hoarding for no good reason.
You should now have a nice neat pile of clear folders with paper in. Put them in box files - one (or more) per person. Put the box files in one of your big stackable boxes and put them somewhere.
Go and have a cup of tea and a nice cake.
Are all the surfaces clear? No? What else is there? Books - decide where they are going to live. If it's not this room put them in a box for now and put them where they live. Ditto CDs/DVDs/computer games.
Now go and look at the furniture. Do you like it? No? Arrange to get rid of it - if you think it might be worth something then sell it, freecycle it or take it to the dump. If you like it - do you need it? That might not be something to answer until you have done more rooms. If it's in bad condition arrange to get it fixed or put it straight on the junk pile.
You now have one room clear. You can make this your organisation room - find/buy a bookcase to put all your nice box files on.
Repeat for each room. Throw away as much as you can (bear to)..and get nice places to put everything you need to keep! Storage is your friend! And labels - otherwise your DH will use you as a living authority on where everything is!
Once you have organised everything you will find it so much easier to keep clean and tidy I promise! Then your routine needs to be "always put things where they belong as soon as possible" and you'll be able to stay on top of things!
Good luck!