Try this and adapt it to your need:
function Validate()
{
var message = "";
var ddl = document.getElementById("<%= ddlFinalAction.ClientID %>");
var ddlValue = ddl.options[ddl.selectedIndex].value;
if (ddlValue == "")
{
message +="Must Enter Final Action Taken\n";
}
var txt = document.getElementById("<%= TextBox5.ClientID %>");
var txtValue = txt.value.trim();
if (txtValue.length == 0)
{
message +="Must Enter Comment";
}
if(message != "")
{
alert(message);
return false;
}
else
{
alert("ok");
return true;
}
}