how can we use javascripts in visualforce pages?

<apex:page sidebar="false" showheader="false">
<script>
function changeFontColor(Id,textValue) {
 if(Id.checked) {
  document.getElementById(textValue).style.fontWeight = "bold";
  document.getElementById(textValue).style.color= "green";
 } else {
  document.getElementById(textValue).style.fontWeight = "Normal";
  document.getElementById(textValue).style.color= "black";
 }
}
</script>
 <apex:outputPanel layout="block">
  <label for="checkbox">Click this box</label>
  <input type="checkbox" id="check1" onclick="changeFontColor(this,'{!$Component.theOutputPanel}');" />
 </apex:outputPanel>
 <apex:outputPanel id="theOutputPanel" layout="block">Change Font Color and Font Weight</apex:outputPanel>
</apex:page>

Demo Link : http://sfdevforce-developer-edition.na12.force.com/JSinVF