We pronounce both of these words the same way, which is why you probably misspell one of them every time you write it. And because they’re both real words, it’s the kind of mistake your spell checker probably won’t pick up.
Stationary is the opposite of mobile. It’s an adjective describing a thing that doesn’t move around.
Stationery is the fancy paper with your family crest stamped at the top. You keep it in a desk drawer in the drawing room of your 17th century estate, writing letters to your long lost lover who’s gone off to fight the war. “Why must men squabble and fight this way,” you ask, “leaving behind their desperately anxious maidens to wonder if they’ll ever marry?” Then you splash a little perfume with the print of your lipstick and hope that he’ll smell you over the stench of gangrenous limbs from his cot in the medical tent. That is, if the mail carrier can make it to the front lines alive.
Perhaps it will help you to remember the correct spelling of stationery by trying to pronounce the word with a British accent, as though you’re requesting that your butler fetch it for you.
Good luck and remember, no matter what you do, you’ll do it better with proper grammar.