It is plural because it has two blades. If you want to make it singular, then use "a pair of scissors". Interestingly, where I am now (Ireland), I've caught people saying "Do you have a scissors?" I suppose this is just a local way of speaking, as it is definitely plural in every standard form of English.

plural

It is plural, but a pair of scissors is a singular object. It's confusing, I know, but no one talks about just one scissor.