The strings that normally come with your ukulele will probably be a load of old rubbish. Why are they rubbish? There are two main reasons - they sound dull and do not "ring" when plucked or strummed and, probably more of an issue at this point, they stretch and go out of tune all the time.
The strings you should be looking for are either Aquila strings made of nylgut or flurocarbon strings (good quality fishing line) the best being made by Worth. Worth strings are twice the price of Aquilas but are twice the length allowing you to get two sets per packet.
Make sure you buy for the correct size of ukulele.