For some, all that is needed is to be white and not overly ugly. Experience helps, but I imagine it is seldom checked. There are plenty of English teachers and tutors living quite comfortably in Asia (including Hong Kong) with no more than this.
Academic or professional qualifications will depend on what age groups you'll be targeting. If you want to cast as wide a net as possible, then you may wish to take one or other of the two most widely recognised EFL teaching qualifications : Trinity CertTESOL and Cambridge CELTA. On top of this, experience is a definite advantage.
Each of those courses is about a month long (full-time), so if you want to get qualified before summer then you'd better get started. It is possible to study for the Trinity CertTESOL here in Hong Kong, but it's just about the most expensive such course in the world.
If you want to do things strictly 'by the book', then you will need to either register with a tutor agency (who'll keep you busy but will also keep a large chunk of your earnings), or you'll have to register a sole proprietorship. This is quick and simple to do and keeps you on a sound legal footing.
Many don't bother with the legal niceties, though, and most seem to get by without any trouble at all (unless they start tutoring the child of a Government official).