After about three months learning to play the mandolin, I saw a guy in a lesson video whose mandolin sounded so much nicer than mine and it is the same model mandolin.  Both instruments are Kentucky KM 150S A model mandolins.  I wondered how he got his mandolin to sound so good and it turns out that the difference is in the strings.  If you are playing a new mandolin, with the original factory strings, chances are good that you are playing on inexpensive heavier strings.  The mandolin is a delicate instrument that can resonate a beautiful sweet sound.  I am learning to play bluegrass music and after a little research, I found that many bluegrass mandolinists prefer light-medium weight strings.  The best quality ones I have found and the ones that I am currently using are Black Diamond model N762 silverplated wound loop end strings.  The weight of these strings are:

G string .035 inch

D string .022 inch

A string .012 inch

E string .0095 inch

These are the sweetest sounding strings I have ever heard.  In fact my father only uses Black Diamond Strings on his Banjo as well.  They have such a marvelous ring to them.  Now don't let the name silverplated scare you.  These strings sell on the internet for about $7.95 a set.  There are comparable strings in other brands.  The important thing is to choose light-medium strings.  They add an ease of playability as they require much less finger pressure to fret.  The sound is fantastic and it is much easier on the beginner player's fingers. 

One word of caution.  Thinner strings may sit closer to the fretboard than the old thicker strings.  When I added these strings to my mandolin, i had to raise the bridge slightly to correct an ugly buzz that developed right after changing them.  So add a great set of strings, adjust your playing action and tune them in for the sweetest mandolin sound you've ever heard.  Hope this helps.