The "Standard C" tuning (sometimes referred to as "Re-entrant tuning") is the most common ukulele tuning for soprano, concert and tenor ukes. Standard C tuning features strings tuned (from fourth to first strings) to G C E A. Guitarists new to the ukulele are in for a surprise, however, as the pitch for the open strings in C tuning does not progress from low to high, as it does in traditional guitar tuning. The lowest string on a ukulele in C tuning is tuned to a high G - the second highest sounding open string.
Because of this unfamiliar tuning, it makes sense not to tune the lowest (fourth) string of the ukulele first, as you would on a guitar. Instead, begin your tuning with the third string of the uke, which is the note C.
If you have access to a piano, find and play the note "middle C", and tune your ukulele to that. To find the right pitch for this open C string using a guitar, reference the first fret on the second string of any in-tune guitar, and adjust your uke tuning to that note. If you have access to a chromatic tuner, tune the third string on the uke to C. Or, you can simply listen to this recording of an open C string on the ukulele.
Once you've got your C string in tune, you can use this note to tune the rest of the instrument. The open second string of a ukulele is E. To tune that string, press and play the fourth fret of the third (C) string on the ukulele, which is note E. Now adjust tuning on the E (second) string until the two notes sound the same.
Using your freshly tuned E string, you can now tune your lowest string - the G string. To do that, hold down and play the third fret of the second (E) string on the ukulele, and tune your open fourth string until the two notes sound the same.
Lastly, tune your first string - the A string - by holding down the second fret of the fourth (G) string. Now, adjust the tuning on the first (A) string until the two notes sound the same. At this point, you should be in tune. To double check your tuning, listen to this recording of all four open strings being played on the ukulele.