Ariel's gender has been the subject of much debate; it is now generally assumed that the spirit's gender is so unimportant to his role in the play that Ariel can be played by either a man or a woman. For much of the play's history, however, Ariel was always played by a woman. After female players were introduced in the mid-1600s, a female actor would embody Ariel. This continued until the early 20th century, at which point custom shifted. In some high profile recent productions of The Tempest, Ariel has been played by men (Colin Morgan, Ben Whishaw) but has been given an androgynous presentation.
Part of the confusion arises from the lack of pronouns used to describe Ariel in the play. There are two uses of "his," once in a stage direction and once in Act 1, Scene 2, "Ariel and all his quality." But, given that "his" has long been used in English to mean "one's" or "his or her" -- as in, "to each his own" -- this is not definitive. Ariel could be a genderless spirit who uses male-default language in this instance.
So, Ariel might be male, or female, or genderless. It is up to you or to the director, and doesn't make a significant difference either way.