Before playing songs you should learn how to play basic chords.
Here is what ukulele chord boxes look like:
The virtical lines represnt the strings where you will place you fingers. The horizontal lines represnt frets. Always make sure you place you finger on top of the string as close to the fret without being on top.
The cirles above the chord bow tell you that the string below will be played open.
When looking up chords the number in the circle is what finger you need to use.
Try playing C chord (shown above) by placing you 3rd finger (ring finger) on the 1st string's 3rd fret.
Once you have that try Am, F, and G; basic chords that are used in most songs.