I have a speaker at home but it is not really playing well and a friend suggest I put a capacitor on the speaker for it to play well..

You can use a non polorized elctrolytic in series to pass only the Highs, this way a tweeter won't try to reproduce the lows.
You can place a large capacitor across power input leads, to help hold the voltage when the woofer produces the deep bass.
You just have 1 speaker? Stero can make it sound better, is it in a cabinet?

Thank for the ans,I will try this out and what's up with it... thnks again