Open Source is not a language.
Before even thinking about a language you'll need to learn computer programming. (Otherwise you'll have knowledge of a language, but no knowledge of how to use it. If you learn French, could you then be a French doctor?) Try http://www.amazon.com/gp/offer-listing/0130224189/ref=pd_bbs_sr_olp_1?ie=UTF8&s=books&qid=1212765521&sr=8-1
You should already know HTML and CSS if you do web design (they're markup [the look of the site] languages.) You'll need javascript, php and SQL, for starters. PHP is trivial if you understand programming - you can almost learn it from the manual as you write code. Javascript is a bit more difficult. SQL is a totally different thing - you'll probably need a good tutorial or a good book to even begin to understand how to use it - you have to understand how databases work. (And you have to learn how to design a database, which is another totally different discipline).
Your best bet might actually be to hire a local web developer to develop the site (you can do the design), with part of the contract being that he teaches you how and why he's doing what he's doing. (Once you've learned programming, of course.)
(BTW, AJAX - and you should learn it - is a technique, using javascript and, usually, php, it's not a language itself.)