you might think it's a little difficult, but actually it breaks down to a series fo relatively easy steps: why not try making an "online booking system" for theatre seats?
Step one: figure out how to display the seats in the theatre using a table. (say a 5x5 table, with rows going from A - E and seats from 1-5)
step two: make that table dynamic so that you can tell it how many rows and columns you have in your theatre and it displays the table accordingly.
step three: start's you into databases too! design a database so that you can enter a row and seat and set the status of that seat as free, booked or reserved.
step four: do a form that will allow you to enter information (as above) into the database from your web page
step five: change your seat arrangement page so that it displays differnet seats in different colours, depending on whether they're booked, reserved or free.
etc etc
This is similar to one that i've just completed for booking tickets for a school play, and i found it easy to get into and a good way of trying out lots of stuff. Let me know if you want any specifc help too, since I've just done it all myself!