How to make a script execute on page load?

I am trying to create a HTML script that will pick a random number out of a list I have given when someone loads the page. This means if they refresh the page a new number will appear. I know how to generate a script that shows a random word/number when you press a button but it is necessary to make the number generate itself and also it can't be in a wordbox. If anyone can help me with this, I'd appreciate it.