I'm not being a smartass, but the best language for the job is often the one that you are most comfortable with. If you are starting from scratch (i.e., learning how to program), one thing you might want to look at is the amount of additional software/libraries/etc. that support the specific task you are trying to accomplish.
For example, I do a lot of scripting with python. There are a ton of libraries that make doing web 'stuff' easier. Examples are curl (get/send http and other requests, manage cookies, post forms, scrape screens, etc.) and beautiful soup which parses XML and XML-like languages. With these two things alone, you do 95% of what most people need to do.
Another consideration is scripted languages vs. compiled. For all practical purposes, you can probably ignore the efficiency differences that exist between scripted and compiled languages. Productivity is much, much higher with scripted languages for multiple reasons.
You might also want to consider 'open' languages like C, C++, perl, python, etc., vs. vendor-specific stuff like C#. The beauty of the open stuff is that they are usually easy to move from one OS to another, especially the scripting languages. I have a toolset in python that I can run on windows, unix, linux, whatever, without any changes.
Not knowing much about what you want to do, I'd point you towards python and perl, with python, in my opinion, being easier for a newb to pickup. I'm sure there's nothing wrong with C#; it's just not something that I'll ever have an interest in picking up.
Good luck.
