I see that you are from UK; if you want to come to Japan for less than one year or so, then you might consider Working Holiday Visa (up to either six months or one year), which allows you to travel around in Japan while having a way to earn some money along the way, so that you can experience traveling/living in Japan and getting a feel for teaching. If you are thinking of pursuing a career in teaching English in Japan for longer than that, though, you'll need to complete your university study and obtain a bachelor's degree. That is an Immigration requirement for full working visa for Japan.
If you are thinking of high schools, you might look into JET program. If for college, you will probably be required to have completed higher level of studies and possibly better come equipped with some years of teaching experience already...
Please search for similar threads on this forum for more information :)