suppose the id of the field required to be validated is 
NameId

Thus:- 

function validate()
{
  if(document.getElementById("NameId").value == null)
  alert("Please enter the name");
}

function validate()
{
  if(document.getElementById("NameId").value == null)
  alert("Please enter the name");
  document.getElementById("NameId").focus();
  return false;
}

or 

function validate()
{
   with(document.formname)
   {
     txtboxid.value == ""
     alert("Please enter value");
     txtboxid.focus();
     return false;
   }
}

or we can use 

document.form.txtboxid.value

instead of

document.getElementById("NameId").value 


regards
rajikiran

function validate()
{
  if(document.getElementById("NameId").value =="")
  alert("Please enter the name");
  document.getElementById("NameId").focus();
  return false;
}