Megane 3 Stereo upgrade

hi guys, sorry if this is in the wrong section or already on here but I have been searching loads of various posts online and I can't get a good answer for my question. I have a megane mk3 2.0 tce 180. As your all probably aware the sound quality is pants so I am wishing to improve. I would be interested in fitting an under seat 'hideaway' sub and some door speakers. I know that the door speakers should just be a direct replacement as regards to wiring. But my question is how exactly do you wire a subwoofer in and can it even be done with the standard head unit. I am not sure if it will make a huge improvement because the head unit is apparently rubbish. Can this be replace also?
Any help/advice would be brill

Megane 3 Stereo upgrade