Whether you've just bought an exciting new photograph or have a prized older one already on display, there are things you can do to protect and preserve your precious possession.
First of all, the print should be framed under glass or plexiglass. This transparent cover, generically called glazing, protects the print from dust, fingerprints, scratches, and food fights or other random junk in the air. I prefer glass because plexiglass is a static electricity machine that just loves to attract dust. Plexiglass also scratches easily. But glass is heavier and more dangerous if it breaks, so glaze it under plastic if you want to.
Assuming the print is under glass (or plexi), it must be matted. This means the photo is placed behind a thick piece of specialty paper board, called mat board, which is typically a little over 1/16" thick. The mat board has a window cut out of it to reveal the photo beneath. The mat board spaces the photo away from the glass. If the photo isn't matted, it will be in direct contact with the glass and will almost certainly stick to it over time, ruining the print. The mat board also ensures empty space around the photo to set it off, a more pleasing aesthetic presentation of your prize.
You will want to clean the glazing periodically. Windex or the like is excellent for glass. Just be sure to spray the cleaner on your cleaning rag, not directly onto the glass. If you spray onto the glass there is a chance that a dribble will run down and get behind the glass, migrate onto the mat board, and ruin the framing job. (If it's unmatted, you'll ruin the picture.)
Cleaning plexiglass is a little harder. You must use a soft, soft cloth because plexi scratches so easily. Even paper towels, believe it or not, will scratch plexi. I think I've used Windex in a pinch to clean plexi, but I don't recommend it. Plastic is more chemically active than glass, and I don't know what sort of reactions go on between plexiglass and Windex or other cleaners that might fog the plastic. The best cleaner to use is one designed just for plexiglass. You may have to find an art supply store or a framing shop to find some, but this one trip will provide you with a bottle that amounts to a lifetime supply of the stuff. It cleans plexi incredibly well, and it leaves an antistatic residue as well.
Assuming that you have the photo properly framed and cleaned, what's next to attend to? Light, heat, and moisture. These are the great enemies of photographs. All photographs fade and discolor over time. This may take centuries or only a few years, depending on how the photograph is made and matted. (See my Photo Tip Photo Buying Advice for issues concerning the inherent lifespan of a photograph.) Light (especially ultraviolet light), heat, and moisture hasten this slow degradation.
If you bought the print to enjoy (you probably did), and you know where you want to hang it, you might choose to ignore the light/heat/moisture issue. You might decide that it's better to have the print fade a little sooner in return for displaying it in just the right spot. Depending on the print, we might be talking about shortening the lifespan from many decades to a few decades, and you might not care about the difference. So hang it where you will. Just be aware that if the print is in direct sunlight through a window for large periods of the day, or subjected to high humidity as in a bathroom, or stored in a hot attic, you are shortening its useful life by some amount. (It's hard to know by just how much, there are so many variables at play.)
All other things being equal, try to avoid heat, humidity, and bright light. Think twice, for example, before hanging your treasure in your greenhouse!
Treat your photographs well, and they will continue to please you for a long time to come.