D tuning on the ukulele used to be an extremely popular tuning method, but has recently fallen out of favor in the uke community. D tuning today is most commonly found in England and Canada. The tuning itself is very similar to standard C tuning, except all notes are tuned a whole step (two frets) higher, making the open strings A D F# and B. Let's walk through the steps required to get your uke into D tuning.
As with standard C tuning, it makes sense not to begin tuning on the lowest (fourth) string of the ukulele, because that isn't the lowest pitched note on the uke. Instead, begin your tuning with the third string of the ukulele, which is the note D.
If you have access to a piano, find and play the note D one tone above "middle C", and tune your ukulele to that. To find the right pitch for this open D string using a guitar, reference the third fret on the second string of any in-tune guitar, and adjust your uke tuning to that note.
Once you've got your D string in tune, you can use this note to tune the rest of the instrument. The open second string of a ukulele is F#. To tune that string, press and play the fourth fret of the third (D) string on the ukulele, which is note F#. Now adjust tuning on the F# (second) string until the two notes sound the same.
Using your freshly tuned F# string, you can now tune your lowest string - the A string. To do that, hold down and play the third fret of the second (F#) string on the ukulele, and tune your open fourth string until the two notes sound the same.
Lastly, tune your first string - the B string - by holding down the second fret of the fourth (A) string. Now, adjust the tuning on the first (B) string until the two notes sound the same. At this point, you should be in tune.