Domain- is that the start?

Hello, i am a student programmer and as part of an open project i have chosen to do some UML modelling, none of my tutors can actually help very much as neither of them are very familiar with the modelling technique so i have looked here for some pointers.
I am currently reading a UML book by Leon Starr to get me started and i sort of understand the technique, but i don’t quite know where to start.
I am proposing to model my local cinema(not any online versions-althoughthey do take online bookings, does this mean i have to consider this?)anyway, i am sort of stuck in defining the domains, i guess my system will have the user interface(where a cinema worker deals with the request for tickets) a screen domain (as different screens may show different films), but now i get lost???