'scissors is on the desk' is it right?
No, it is not. The word "scissors" is a noun (and although it
ends in an S, it is singular); in the sentence you are asking
about, it would need an article. There are two ways to do this: if
you are pointing it out by name, you would say, "It is a scissors."
(It's not a rock, nor a tree, nor a knife. It's a scissors-- that's
what it is called.) But if you are differentiating one pair of
scissors it from others, you would say, "It is the scissors I
borrowed from my sister."