Sneezing could be anything from simply inhaling dust up the nose, or be be the start of an upper respiratory infection or any number of things.

If the cat is eating, playing, sleeping normally and has normal litterbox habits I wouldn't worry too much about it if the sneezing just started.

However if it continues for a week or more, cat starts sleeping more, vomiting, stops eating or using the litterbox, or has any kind of discharge from the eyes/nasal passages, then get your cat in to see the vet.