Try some online tutorial available on the net, you may also take the help of this one which I found a good one to start with JavaScript,just go through it from here, JavaScript Tutorial
Douglas Crockford has a video series that introduces javascript. It may be a little technical for beginner programmers, but if you are comfortable with code in another language you can find the videos through the YUI theater page. Scroll to the section where it says "Douglas Crockford — The JavaScript Programming Language" Here is the link: http://developer.yahoo.com/yui/theater/
I will probably be shot for saying this, but learn jQuery. jQuery is a JavaScript framework and its use is gaining traction quite a lot. It is used by a lot of sites and eventually it will become a must have skill to be a web developer in a few years (imo, it already is). Learning conventional JavaScript has its advantages too though.