What do you mean exactly?
Just a button that goes off to another page? You can style one up with CSS, or Google for button generators.
Or maybe you mean a button that, when clicked, reveals some extended text, maybe in a dropdown/sliding motion? That would be done with JavaScript. You could do it with jQuery (might be including in your WordPress theme already) – Look into the slideUp()/slideDown() methods. You might also find other scripts online under the umbrella of “content accordion” or the likes.