First off, you are not alone. Take a deep breath, there are millions of your sisters who've made progress. It just takes some thinking and some planning and some work.
Before you do anything else, take paper & pencil & go make your bed. Don't worry about dirty sheets, just make it. That gives you 30 square feet or so that's neat. Now sit there on the bed and think. Five minutes: How much time do you have? How critical is this visitor? What is this visitor likely to want to see and do? Then make notes about what your actions are and get to it. But don't forget to read the rest of this page.
How much time do you have?
It's helpful if you have a timer. If you really attack energetically, you can do a lot more than you think in 10 or 15 minutes. Having a timer forces you to focus on the most visible stuff. Set it for 10 minutes, and switch areas as it dings. If you get extra time, you can go back to an area. Divide the time up based on the proportion of work you think is in that area to the total time. So if you have 30 minutes, half the work need to be done in the living room, you set the timer for 15 minutes for pick-up, 10 minutes to pick up & clean the hall & bath, and 5 minutes to do a quick vacuum "down the middles".
If D-time is in 30 minutes, clean the living room of all trash and ongoing projects, dust and vacuum there, foyer, hall & the guest bathroom. Stash all keepables in a trash bag, you can sort them later. (Just don't mix up the keepable bag and the trash bag!) Hide bag in empty room, under couch, deep in the closet, anywhere. Put trash outside. Now clean guest bathroom. Light nicely scented candle. To keep visitor out of kitchen, if expected to serve refreshments, turn on kettle to make pot of tea, or make coffee and put in server, grab the Girl Scout Cookies out of the freezer and put on plate, and have tray ready with cream & sugar (or iced tea pitcher, glasses, lemon & sugar/substitute) to bring in.
If it's an hour, clean the kitchen counters & sweep the floor, too. Dining room is optional. If it's two hours, clean the kitchen first, then mop the floor, tackle the bath, the living rom & the dining room while the kitchen dries.
If it is a matter of days, not hours, then you need to be more systematic.
Find four boxes. one for trash, one for stuff to give away or yard sale, one for things you're keeping but belong elsewhere, one for stuff you're not sure about, plus an envelope for bills & urgent papers & one for miscellaneous, not trash papers. As you go, put an item of clutter in one of the four boxes (line trash with plastic bag to save boxes; also can do with give away box). The stuff you're not sure about, close the top, write on it the contents, date it and throw away anything you haven't looked for in 6 months. That will let you stay in one place while cleaning, then when you're done with the room, you can put the strays back in the proper corral.
You're in panic mode so don't try to file your piles of papers. Do sort out the bills and other important mail and put all the rest of stray papers ( toss the obvious junk mail) in a box and sort later. Same applies to any collection of toys, parts, crafts, puzzles, etc. Put obvious projects together (plastic ziplocks, esp the gallon size, work wonders, on a short term basis), then put everything else in a box or bag to go over later.
Where to start? At the front door. Stand there and decide what your guest will see from the door and work on that first. Declutter, then sweep or vacuum and dust. Then go to where your guest will go, and clean that area, then any "lines of sight" from that area. Trace the path to the guest bath, & clean up along that path, plus any "lines of sight" you can't shut a door on. Then tackle the guest bath. By "lines of sight" I mean the area visible from one room into the next: thus if your living room opens on the kitchen, clean the stuff visible from the living room, but not the stuff on the wall next to the living room. Be sure to catch all the angles. Once you've done that, you can go clean the areas they might go in, kitchen and family room being first, then dining room (unless you're serving dinner...).
If your visitor is staying overnight, the guest room should be organized next. Remove all items that do not fit under the bed, make sure the sheets are clean or start washing them, vacuum & dust, put up a suitcase holder & make room in the closet to hang clothes and a drawer in the dresser. If you have those little hotel samples (or have time to check out the "personal sizes" at the grocery or drug store), make a basket with toiletries, some dry snacks, a bottle or two of fancy water and a nice glass. I am seriously considering putting towel racks on the backs of our bedroom doors so the bathroom would stay clear of wet towels, and add a little extra humidity to our winter-dry rooms. And the kids wouldn't use the nice towels you bought for guests.
If in spite of all your hard work (or due to major outside interference) you aren't ready and your MIL is the critical sort (or even if she isn't), ask her for her cleaning & organizing tips and write them down. Earnest student is an easier stance than resentful correctee. You don't have to do what she says after she leaves, after all.
And now that the crisis is over...
Go back and sort the boxes and containers you stashed stuff in on an emergency basis. After all, the Legos, dolls, pens, paper clips, puzzle pieces, and cash receipts deserve to be in their proper home. Even if that home is the waste basket. Besides, unless you mend your ways, you'll need those stashing places for the next unexpected visitor.
The time has come to worry about long term cleaning plans. Make a card or list for the daily maintenance stuff, like make bed & clean counters. Speaking of which, those two things make everything else, even mess, look much better. Just making beds gives a better atmosphere. So while you're cleaning, make a note to remind you of what needs to be done on a daily basis, 10-20 minutes worth (w/o interruptions) in the AM and the same in the PM. I make bed, put away/wash breakfast dishes, spot clean bathrooms (wipe up spills & whiskers, put up toothpaste before they harden, swipe toilet back & rim with wad of tp or baby wipe before misses turn into yellow crystals), sweep kitchen floor in the morning, and try to pick up toys (or help kids to, hey they gotta learn, right?), do after supper dishes & pans (start dishwasher, then put up while doing morning coffee or tea). If you have a lot of laundry, you might want to add "wash one load" in the AM, "dry" in the PM or vice versa, whenever you have time to fold or hang the clothes. To do one load only takes a a few minutes.
When your adrenaline is flowing, you can do marvels. But a few minutes every day can make crisis cleaning a whole lot easier. So a few simple actions can prevent a lot of major hassle.
Well, I hope these ideas have been helpful. Please let me know if you have any suggestions or improvements at the e-mail address at the bottom of the page..