As was mentioned, you will need to get your hands on two pieces of software. The first is called Sphinx-2. Sphinx-2 is a speech recognition engine. The second piece of software you need is called Festival. Festival is a speech to text recognition engine.
Installing Sphinx-2 is relatively easy to do. It has binary set up for most major flavors of Linux. That means all you will have to do is find the right one, download it and deal with the questions that the wizard asks you. Nice and easy. So simple that you could even let a Windows user do it for you without fear.
Next up on your listing is installing Festival. That is a bit more complex. You need to get Perlbox if you do not have it already installed. You will need to install Festival only after you put in Perlbox. Your installation specifics will depend on your flavor of Linux. Also be aware that you need to run Perlbox before you open Festival. The command is perlbox-voice.