The strings on a ukulele are numbered from the bottom to the top.  Hold the neck of the  uke in your left hand, so the head of the uke is pointing left.  The string that is on top is string number 4.  The string that is on the bottom, farthest away from you is string number 1.  It is important to learn to number the strings correctly so you will have an easier time learning notes and chords on the fretboard.  

 

The stings are named after the pitch that they are tuned to.  Going from the top to the bottom, string 4 is G, string 3 is C, string 2 is E and string  1 is A. A common way to remember the string names is to make up a little sentence using the names of the strings for the beginning of each word.  So, for G,C,E,A you might use Goats Can Eat Anything, or Good Children Eat Apples.