'08 G6 Base Model sub installation

I have a 2008 G6 inline 4 that I want to install a sub system in, but I'm working with limited money, so I want to find a decent system for fairly cheap (below $250), and I need to know if I will have to install a new radio headunit as well. If it is possible, I'd really appreciate a slightly in depth tutorial on how to install the entire system, considering I've never installed an audio system in any car. I've had the car for around 2 months, still paying on it, which is why I'm working with such limited money. Come January, I'll be taking a new job making beaucoup bucks, so then if any modifications need to be made they will be taken care of rather quickly. Thanks