Each type of ukulele has different tunings. If you're not sure what type of Ukulele you play, check out of guide to Ukulele Types.
Soprano ukuleles are most commonly tuned in C - high G-C-E-A where the high G is the string closest to you when holding the uke in the playing position.
They can also be found to be tuned in D - high A-D-F#-B from low to high. Generally I'd recommend that as a beginner you tune to C though as this is the key that most uke tabs and chordsheets are in on the web.
Tenor ukes tend to be tuned in the same C as sopranos - high G-C-E-A from low to high.
It may seem odd that the Tenor, typically a lower pitched instrument, has the exact same tuning as the Soprano uke, but the longer scale length of the Tenor make the strings more tort and allows for more finger space which can accomodate faster playing. It's also less cramped and therefore more comfortable for a lot of players, especially those with larger hands.
A Baritone ukulele is typically tuned in G - D-G-B-E from low to high.
This tuning will be familiar to guitarists - it's the same as the top 4 strings of a guitar. This can make the transition from guitar easier, but you may find it less rewarding. Baritones don't have the same famed ukulele sound that the higher tunings have.