You can use a keyboard or piano to tune each individual string on your ukulele to a specific note. It is pretty easy and great for beginners because you are only playing open strings on the ukulele, and so you can have a free hand to turn the pegs while the piano and the ukulele are resonating. It's also good because you tune each individual string, instead of getting one reference note and tuning the rest of the strings to it - so on the whole it is more accurate (as long as your piano is in tune!)
The picture above shows you each string on the uke, and how it corresponds to the same note on the Piano or Keyboard. Just follow the picture for each individual string, play the note on the piano or keyboard and tune the string to that note until the two notes match. Make sure you use the Middle-C on the picture as a reference point so you don't try to tune your ukulele an octave too low, or too high. If you are using a piano, hold down the foot-pedal so the note you are playing rings out for longer. This should make matching the note to your ukulele a little easier.
Unlike on most other stringed instruments, the order of the notes from left to right on the keyboard does not correspond to the order of the strings on the ukulele. Start with the C-string first, and keep an eye on the picture above to make sure you are working with the right note and the right string.
To see other methods on how to tune the ukulele visit our How to Tune the Ukulele page.