How to unstick command, option, shift, and control keys on a mac?

I just bought a new mac, and although it didn't do this at the start, it is now continually making noises every time I press the shift, control, option, or command keys. Besides that, an irritating icon pops up somewhere on my screen like an advert. If I press a key once while pressing another key simultaneously (say shift+a to get a capital A), the key functions normally. But if I press the same key twice in quick succession, it'll stick (so if I press shift twice quickly, all the letters I type will be capitalized). It's really bothering me, and I have no idea how I triggered it or how to get rid of it.