When you open the Ukukele Chords widget it will show you how to easily play a C major. Change either the note (A to G) or the chord type (e.g. Minor) and the display automatically updates to show you the different ways you can play the chord.
The most common tuning (G C E A) is preset by default. If your ukulele has a different tuning, click the (i) icon in the bottom right corner and change each of the string tunings accordingly.
You can also tick the box to see crazy unplayable chords -- but you would need three hands or impossibly long fingers to play those, so this is just for curious folks!