Take a look at Eclipse (as mentioned by January). You can use Eclipse to develop in most of those languages above (I've used it for Java, C++ and Ruby).
For Java development you'll also want to make sure that you have a JDK (Java Development Kit) installed. The easiest way to tell if you have one, is to run the following command from terminal:
$ javac -version
And that should return something like this:
Setting up a Ruby development environment can be a little tricky. Here's a tutorial on how to specifically setup Ruby on Ubuntu.
However, for .Net development you will want a Windows instance with Visual Studio. The Mono project has come a long way with building an open source implementation of the .Net framework. But (especially if you are a comp-sci student) you will want to make sure that your local .Net framework matches-up with whichever one your class will be using. While Visual Studio is expensive, the Express Edition should be available by free download, or your professor may have a suggestion on how/where to get a full version.