Sound system upgrade, suggestions?
 

Hi.
I have just bought an audi a4 cabriolet 2.0tdi sline 2006, beautiful car in and out.
Problem is I don't really like the sound, and one of the speakers seem to be cracked.

I don't understand or have any knowledge about speakers and amps and stuff. But basically I want a system that will sound better than what my friend has in his audi, which is the bose system.

So can anyone tell me what I need to buy, including the speakers siZe, and rough price for each component. Oh and I don't want to mess with the boot space, so and underseat base speaker would be preferred.

Thank you