The ukulele has 4 strings:

A|-----------------
E|-----------------
C|-----------------
g|-----------------

Above is a representation of the ukulele in tab notation. In tab, the letters represent the strings, and the numbers represent frets you're supposed to play. When holding the ukulele, the A string is the one closest to the floor, and the g string is the one closest to your head.

   C   c d e f g a b c

A|-3-|-----------0-2-3-
E|-0-|-----0-1-3-------
C|-0-|-0-2-------------
g|-0-|-----------------

The tabs above show 2 measures. The first one is a C major chord. The second one is a C major scale. Try them out!

The notes of a major chord come from the major scale. The first note of a chord is called the root note. This note gives the chord its name. The other notes are called the 3rd and the 5th, because they are the 3rd and 5th notes in the scale that the chord is named for. In a C major chord, E is the 3rd and G is the 5th.

   F   Am

A|-0-|-0-
E|-1-|-0-
C|-0-|-0-
g|-2-|-2-

The C major scale defines the key of C. In a key, chords are named for the position their root has in the key. F major and A minor are the 4th and 6th chords in the key of c.

With these 3 chords, you can play Wake Up by the Arcade Fire!

Wake Up - Arcade Fire

   C
A|-3-3-3-3-3-3---3-|-3-3-3-3-3-3---3-|-3-3-3-3-3-3---3-|-3-3-3-3-2-2-2-2-|
E|-0-0-0-0-0-0---0-|-0-0-0-0-0-0---0-|-0-0-0-0-0-0---0-|-0-0-0-0-0-0-0-0-|
C|-0-0-0-0-0-0---0-|-0-0-0-0-0-0---0-|-0-0-0-0-0-0---0-|-0-0-0-0-0-0-0-0-|
g|-0-0-0-0-0-0---0-|-0-0-0-0-0-0---0-|-0-0-0-0-0-0---0-|-0-0-0-0-0-0-0-0-|

   Am
A|-0-0-0-0-0-0---0-|-0-0-0-0-0-0---0-|-0-0-0-0-0-0---0-|-0-0-0-0-0-0-0-0-|
E|-0-0-0-0-0-0---0-|-0-0-0-0-0-0---0-|-0-0-0-0-0-0---0-|-0-0-0-0-0-0-0-0-|
C|-0-0-0-0-0-0---0-|-0-0-0-0-0-0---0-|-0-0-0-0-0-0---0-|-0-0-0-0-0-0-0-0-|
g|-2-2-2-2-2-2---2-|-2-2-2-2-2-2---2-|-2-2-2-2-2-2---2-|-2-2-2-2-0-0-0-0-|

   F
A|-0-0-0-0-0-0---0-|-0-0-0-0-0-0---0-|-0-0-0-0-0-0---0-|-0-0-0-0-0-0-0-0-|
E|-1-1-1-1-1-1---1-|-1-1-1-1-1-1---1-|-1-1-1-1-1-1---1-|-1-1-1-1-3-3-3-3-|
C|-0-0-0-0-0-0---0-|-0-0-0-0-0-0---0-|-0-0-0-0-0-0---0-|-0-0-0-0-0-0-0-0-|
g|-2-2-2-2-2-2---2-|-2-2-2-2-2-2---2-|-2-2-2-2-2-2---2-|-2-2-2-2-2-2-2-2-|

   C
A|-3-3-3-3-3-3---3-|-3-3-3-3-3-3---3-|-3-3-3-3-3-3---3-|-3-3-3-3-2-2-2-2-|
E|-0-0-0-0-0-0---0-|-0-0-0-0-0-0---0-|-0-0-0-0-0-0---0-|-0-0-0-0-0-0-0-0-|
C|-0-0-0-0-0-0---0-|-0-0-0-0-0-0---0-|-0-0-0-0-0-0---0-|-0-0-0-0-0-0-0-0-|
g|-0-0-0-0-0-0---0-|-0-0-0-0-0-0---0-|-0-0-0-0-0-0---0-|-0-0-0-0-0-0-0-0-|

Tab doesn't really have a good way of representing rhythm, though I've tried to arrange the notes so that it gives a rough idea. Listen to the song while looking at the tabs, and then try it out!