Every ocarina is different and unique. There are so many of them of various shapes and colours, as well as the specific tunings (the note an ocarina starts on) - such as C, F, Bb, Eb, etc.
But the beauty of this instrument is that it's fully chromatic, which means though it may be tuned differently, it does not really have a specific key. Just like on the piano! So at the beginning, you don't really need to care if you have a G-, D- or F-ocarina - you can still play all your favourite tunes in C.
When you feel ready, you could play in the key your ocarina is tuned to, using its specific fingering system, as that will unlock the full potential of your instrument. You will even be having several versions of sheet music to fit the range of your ocarina better.
What can work amazingly for beginners is the Fingering Chart. Even though various ocarinas may be tuned differently, the charts will work all the same.
You can start with the basic C Major Fingering Chart above, which perfectly fits 4, 5 and 6-hole ocarinas. The only difference is that a 4-hole simply doesn't have the 2 highest notes - D and E, and a 5-hole won't have a high E.
Try to learn and memorize the finger combinations and then train your fingers to play the fingerings gracefully.