As one of the comments asks, what's wrong with the floor? It looks pretty good. If you're just wanting to add a new coat of finish, use a fine grit (120 or even higher) to scuff up the floor before laying down a new finish, otherwise it won't adhere properly.You might even be able to get away with a circular floor buffer with a scotchbrite pad (probably green or gray).
If you really need to sand the finish off because there are damaged areas we're not seeing in the photo, proceed carefully. The square buff floor sander from Home Depot (or other sources) will almost certain prove unsatisfactory for anything other than just the scuffing suggested above. You will need to use a professional class drum or belt sander. Think twice before doing this yourself. There are very hard to control. Even pros leave the occasional ripple in the floor and many first-time users leave the floor worse than when they started.
I would suggest trying to find a U-Sand floor sander. These sanders contain four 6" random orbit heads, making them far easier to control. They're as heavy as a pro drum/belt sander and having honking big motors, so they cut reasonably aggressively. Because they have flat contact with the floor, you are far less likely to leave ripples or ridges (though I'm sure it can be done). The U-Sand sanders use standard 6" hook and loop disks, which will probably prove to be more economical than the oddball size drums/belts you'll have to buy from the rental place if you use a drum/belt sander. The USand website has a search page for rental centers that carry their products. Don't be surprised if you can't find one close by, though.
Make sure you have a clear understanding of the rental shop's policy on unused sandpaper. You don't want to have to drive back to the store halfway through the job to get more sandpaper, but you don't want to be stuck with unused sandpaper, on the other hand. In my not-very-recent experience the rental shops I used gave 100% credit on unused abrasives, but make sure this is clearly stated on the rental contract.
And just to reiterate what multiple answers have already said - do not start with 20 grit unless you are trying to remove heavy damage. Even 36 grit is aggressive unless there are deep scratches and dents you are trying to remove. If the photo you provided is any indication of the condition, I'd suggest starting at 60 or even 80 grit. If that's not aggressive enough, step back down. Starting with too fine a grit wastes a little time, but does no damage. Starting with too coarse of a grit will cost you LOTS of time to correct and may cause damage you won't be able to sand out.