I created a storyboard on a control in expression blend, but I don't know how to launch it automatically (I mean when the event "pressed", "click" or something else is triggered) without going through the code behind.
Does anyone know how to do this?
You just answered it in your question :) The things you need are Triggers (or maybe Behaviors if you want to do more than just catch an event).
You can read more about Triggers on MSDN
Blend offers a quite userfriendly UI for them too. You can find the Triggers tab on the top-left toolbox.