How to pick 30 questions from 500 possible questions

Hi and thanks for reading!

I am developing my first web app with the help from JQuery Mobile framework. The thing is a exam simulator(multiple choice) in which, after a main menu, the app sequentially shows to the user the questions and the user pick between three posible answers. The user have to answer 30 questions, picked randomly from, lets say, 500 possible questions. And at the end i display the results (% of questions answered right).

The problem is that i don't have idea, conceptually i guess,how to do the picking randomly 30 questions part (i guess that every question will be a jquery mobile page?) from a repository (?????) of 500 possible questions.

Any help or orientation will be much appreciated.

Thanks!

PD: Sorry for my bad english