OK.. This REALLY depends on the platform that you are writing for, and the purposes you are trying to accomplish. This being stated, here is my opinion of three of the leaders in this poll (I have no idea about Lua or Ruby... looked over Lua, its syntax seems weird... no experience with Ruby):
Perl is quite powerful, but its syntax is a mess. It hails back to the early days of Unix programming, and unfortunately, the syntax shows this badly. Imagine, as a previous poster stated, writing most of what you do as a regular expressions, rather than working with arrays and objects. It does boast a large, dedicated community, but this is mostly because it is as old as dirt, and a LOT of people grew up on it. Primarily, only really usable for Unix based environments.
JavaScript is a bit of a pain, but not as bad as Perl. The worst issue with JavaScript is that it can produce some maddeningly invisible errors... invisible, as in that when you run it? NOTHING HAPPENS. Right... nothing. No error code, no "You forgot a bracket on line 23"... nothing. This is especially problematic when there are multiple interpretations of JavaScript, depending on the browser and/or environment it is run in. Couple this with severe limits to its capabilities, such as not being able to write to the file system (at least on client machines... I have no direct experience with JS in server environments), and you have a non-starter.
Python is extremely powerful, especially for math. It doesn't use brackets (like C, C++, JavaScript, etc), yet the syntax is still pretty familiar for anyone knowledgeable in those languages. There are a few issues, here, though as well, such as picking what version to learn. Version 2.7 is the most widely supported, but is inconsistent, version 3.1 has corrected this, but is incompatible with many libraries, still. A more minor annoyance, which might not affect you, is the fact that there is no means in the language to declare anything private... you basically just label it private by a naming convention, and hope others respect that. Python is also available on many, many platforms, and is pretty consistent across them. You can pick up python 3.1 on a local windows machine, for example, and still be able to run the code with little or no modification on a Linux server (although path names may be a pain due to platform dependent file syntax).
As a general script language, Python 2.7 is my recommendation. You can pick it up, along with IDLE (its built-in IDE) from python.org. In the next year or so, expect version 3 to overtake it, but the differences, by then, should be easy enough to handle.
As for learning the language? No idea where to go... sorry. I learned it from the documentation included with Python, but its 'tutorial' is written more akin to a poorly indexed reference document. I would probably recommend something from O'Reilly or No Starch Press, but, again, I taught myself, so don't have any useful recommendations for specific books.