One way is to buy a pitch pipe and twiddle the tuning pegs on your uke until the notes of the four strings sound at the the same pitch as those of the pitch pipe. The four strings of the ukulele are often tuned to the notes A D F# B or G C E A. (The string nearest your nose if you're right handed being A.) The relationship between the strings in each case is the same but the actual pitch is different. In the following, we're using G C E A as the default, but A D F# B is only one tone higher.
Perhaps you don't know much about music. Try this: If you can find "middle C" on a piano, the notes to the right of it are these notes: D E F G A. The black note to the right of F is F# (F sharp). Tune the relevant strings to the appropriate notes.
Haven't got a piano or a pitch pipe? Try this: Tune the "C" string to a note that isn't so high that the string is too tight, but not so low that the string is floppy and without pitch or projection. Then taking the note of the string as the first note of the tune "The Holly and the Ivy", tune the other strings like this: G (Ivy, first syllable) C (The Holly and) E (Ivy, second syllable) A (the). Maybe this is clearer:
(C) The Holly and
Getting the hang of this means you can tune "by ear" without needing a pitch pipe. If you want to play with other fixed pitch instruments you'll need to have an accurate note to start from, but if you're solo, or with other tunable instruments you can tune to any notes as long as the pitch relationships between the strings are accurate, ie if you can play "The Holly and the Ivy" on them.
As alternatives, you could use
The (C) Camp (E) bells (A) are (G) Coming Hurrah Hurrah
Put your (C) glad (E) rags (G) on and (C) join (E) me, (G) hon,
We'll (C) have (E) some (G) fun (A) when the clock strikes one,
English ukulele tuning is said to be adf#b, whereas American tuning is gcea. Another good tuning is Bb Eb G C. Baritone ukes tend to be tuned DGBe.
You can also purchase The Ukulele Orchestra of Great Britain's music on Amazon and iTunes or check out our YouTube channel for videos.