how to validate fields in javascript and enter vaildated data in database(using C# code/codebehind) on single click of an button

<script language="javascript" type="text/javascript">
function Validation()
    {
      var txtfirstname1=document.getElementById('<%=txtfirstname.ClientID%>');
      var txtlastname1=document.getElementById('<%=txtlastname.ClientID%>');
      var txtemail1=document.getElementById('<%=txtemail.ClientID%>');
       if(txtfirstname1.value=="")
      {
      alert("Please Enter Firstname");
      txtfirstname1.focus();
      return false;
      }
      if(txtlastname1.value=="")
      {
      alert("Please Enter Lastname");
      txtlastname1.focus();
      return false;
      }
      if(txtemail1.value=="")
      { 
       alert("Please Enter Email");
       txtemail1.focus();
       return false;
      }
      var s=/\w+([-+.']\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*/;
      if(!s.test(txtemail1.value))
      {
      alert("Enter valid Email");
      txtemail1.focus();
      return false;
      }
}

<asp:Button ID="Btnsubmit" runat="server" Text="Submit"                                      OnClick="Btnsubmit_Click" OnClientClick="javascript:return Validation()" />

protected void Btnsubmit_Click(object sender, ImageClickEventArgs e)
   {

string s = "INSERT into UserDetails (email,firstname,lastname) values('" + txtemail.Text + "','" + txtfirstname.Text + "','" + txtlastname.Text + "')";
                SqlCommand cmd;
                cn = new SqlConnection(conection.getconnection());
                cmd = new SqlCommand(s, cn);
                cn.Open();
                cmd.ExecuteNonQuery();
                cn.Close();
}

<asp:Button ID="btnSubmit" runat="server"                                                             OnClientClick="javascript:return Validate();" OnClick="btnSubmit_Click" />

<script language="text/javascript">
function Validate()
   {
//txtArea is the textbox id
   if(document.getElementById('<%=txtArea.ClientID%>').value=="")
    {
        alert('Please Enter Area Name');
        return false;
    }
    else
        return true;  
   }   
   
</script>

protected void btnSubmit_Click(object sender, ClickEventArgs e)
{
//your code behind code
//inserting the data to database

}