When I was first teaching myself about haiku in 2007, I started with two things. One was The Haiku Handbook: How to Write, Share and Teach Haiku by William J. Higginson with Penny Harter, which I still highly recommend today. It's a book that I dip into a few times a year, and I've even used the lesson plans in it for teaching children haiku. If you're in New Zealand, you can buy it from Fishpond (my favourite online bookstore).
My other main learning tool was a FREE online haiku self study course, which I still recommend to beginners: In The Moonlight a Worm...
You will see on the left hand side under 'The Lessons' section, 'Basho's Spirit', the third one down says 'self study' - just click on that and follow the instructions. There is also a 'Show Don't Tell' lesson with a self study module, which is great learning for any genre of writing.
This self study course is a great introduction to haiku. The website also has information for teachers with lesson plans that you can download, photocopiable poem sheets and a reference section.
I hadn't looked at this website for a while so had forgotten that they had resources for those teaching haiku. I'm going to check out the lesson plans myself now!