You need to learn Javascript before you learn AJAX. AJAX is really a simple set of method calls, the harder part is really understanding DOM manipulation, event handling, etc.
I highly recommend the Sitepoint books -- "Simply Javascript" and "The Art and Science of Javascript." Between these two books, you get a very step-by-step, approachable tutorial of the basics of what you need to start out with.
They teach you in framework-agnostic way, which is important because you want to be able to write JS without relying heavily on any one framework. In fact, one of the books goes into detail teaching you HOW some frameworks work their magic. It's fascinating.
I highly recommend the Sitepoint books -- "Simply Javascript" and "The Art and Science of Javascript." Between these two books, you get a very step-by-step, approachable tutorial of the basics of what you need to start out with.
They teach you in framework-agnostic way, which is important because you want to be able to write JS without relying heavily on any one framework. In fact, one of the books goes into detail teaching you HOW some frameworks work their magic. It's fascinating.
Good luck!