design patterns - How to handle click event in javascript? -

Today is my first day on my new job (a front end lead on a large website). I have a job to implement that button, which triggers the event when I click on it. I did not have any clue, so after a little goggle, I came up with it:

  & Lt; Html & gt; & Lt; Top & gt; & Lt; Script type = "text / javascript" & gt; Function popup (); {Alert ("Hello World") ==> Warning ("Hello World"); } & Lt; / Script & gt; & Lt; / Head & gt; & Lt; Body & gt; & Lt; Input type = "button" value = "click me!" Onclick = "popup ()" & gt; & Lt; / Input & gt; & Lt; Br> & Lt; / Br & gt; & Lt; / Html & gt; & Lt; / Body & gt;   
 The problem is, when I click on my button, nothing happens.  
  Edit  updated on Michelle's comments   
 
 
 This should work  
  & lt; Html & gt; & Lt; Top & gt; & Lt; / Head & gt; & Lt; Body & gt; & Lt; Script type = "text / javascript" & gt; Function Popup () {Warning ("Hello World"); } & Lt; / Script & gt; & Lt; Input type = "button" value = "click me!" Onclick = "popup ()" & gt; & Lt; Br / & gt; & Lt; Br / & gt; & Lt; / Body & gt; & Lt; / Html & gt;