I agree with d'Anconia that you will want to learn some server side programming at some point. PHP is a great language to start with. It has a large community base with in which to learn from. There are thousands of tutorials on the net. Almost all web hosts have PHP installed. Also, it is rather easy to pick up since you already have some experience with JavaScript.
Personally, I wouldn't jump straight into jQuery yet until you feel you have a good grasp on standard JavaScript. Yes, jQuery makes common programming problems elegant and easy; however, you might not have the option to utilize jQuery in every project.
Resources to look at:
http://net.tutsplus.com/
http://www.sitepoint.com/
http://php.net/manual/en/index.php