Running Asp Code

I am very new to asp programing, so this may seem a silly question but here goes.
I am creating a page that will generate random lotter numbers. I have created the actual code to generate the numbers and have added the code to the top of the page. the problem is the code runs when the page loads and I want it to run only on the click of a button.
I read various starter tutorials and none seem to mention how to asign code to the click of a button.