What is your budget?
You can get cheap "flea market" type speakers for a few hundred bucks or high end stuff that will cost upwards of R50K for a set of two speakers.
It is simple case of removing the stock speakers and replacing them with a set of good quality items and will make a huge difference to sound quality as the stock stuff is even cheaper and nastier than flea market stuff.
Polo's actually come with a decent stock system .. 2 way split up front and a mid in the rear ...
depending on how deep ur pockets are ... i'd suggest a 6.3/4 split system up front .. ( JBL/Infinity are reasonably priced for their quality ) and a 2 way co axial speaker for the rear doors, with a matched 4 channel amplifier ... this should set u around 6-8k excluding fitment