Pick one that's generally associated with web stuff. That's pretty much Ruby, Python, Perl, Java, and PHP these days. Take a look at all of them on the surface level to see what's interesting to you, and just pick one and go.

Given that you already know C and C++, I'd learn Perl, Python, or Ruby, since they're a very natural extension of C and C++ and can be used very easily in problem domains where you wouldn't want to use C or C++. Learning more than one of these may prove useful if you want to hack on a variety of different open source codebases, so whichever one you pick first isn't so important as picking one and having it in your toolbox.