you dont need to replace it "correctly" just make sure what you buy will fit the car
if i were you, if you want better sound and quality, go with aftermarket speakers.. they will run fine off your cd player and sound a lot better
___________________________________
well most aftermarket speakers are between 20 rms watts and 150 rms watts depending on size/type and such
most stock units only put out 10-16 rms watts to speakers
you wont have a problem at all.. the only problem that you could possibly face is over-powering them, which you have a 99.999% chance of not doing with a stock head unit, considering they dont put out too much power so you wont have to worry about over-powering any speakers and such
so youll be fine just putting in aftermarket speakers, it will sound a lot better than stock