The zodiac is a ring of twelve signs, and this is the one the ring starts with. Its picture is a ram. Like every sign, it is really a stretch of dates: born in that stretch, this is your sign. Some people believe the sign affects a person's character and life; other people read it for fun and believe none of it, and the word itself takes no side. When you mean the sign, the word is uncountable — my sign is Aries, not my sign is an Aries. The exact dates shift by a day or two depending on where you read them.

Here the sign's name is used for the person born under it. This use is countable: an Aries, two Arieses, all the Arieses in the group. So an article or a plural in front of you means a person is meant, not the sign. Note the article is an, because the word starts with a vowel sound. People use it to introduce themselves, to sort friends into groups, and to joke about how someone behaves. Using it does not mean you believe any of it; it has become a handy label either way.