1. Learn JavaScript (the language)

http://eloquentjavascript.net/ http://www.amazon.com/JavaScript-Good-Parts-Douglas-Crockfor...

2. Learn jQuery (a JavaScript library)

http://jquery.com/ http://www.amazon.com/jQuery-Action-Second-Bear-Bibeault/dp/...

3. Learn SQL (the language)

http://www.amazon.com/Sams-Teach-Yourself-SQL-Minutes/dp/067...

4. Pick an implementation

MySQL, PostgreSQL, SQL Server, Oracle

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After that, most application development languages are just syntax. Pick what feels best.

Learn how to use Ajax to tie client-side requests to server-side requests and you're marketable.