Plays schedule for CNT Teather Split contains a direct link for on-line ticket reservation.
Just click on "On-line reservation" on the play of your choice and pick your seats in the teather to make a reservation, that's right even from your mobile phone :-)
Other tathers don't have on-line reservation, but they also have good plays on their repertoare, so make sure you check all of them before you pick one.

Enjoy!
Croatia-ST