How to prevent people from submitting empty textboxes values into the database.

Hi all, I want to do something like preventing people from entering empty textboxes values into the database. Means to prompt an error message. I also want to do something for my dates whereby date 1 cannot be lesser than date 2. I have already did some of my codes but it does not seems to be working. Why is this so?
ASPX
<center><asp:Label ID="Label3" runat="server" Text="Start Date :"></asp:Label> <asp:TextBox ID="TextBox1" runat="server" style="margin-left: 0px"></asp:TextBox></center> <center><asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" ControlToValidate="TextBox1" ForeColor="Red" ErrorMessage="Please make sure you fill up this textbox before hitting the submit button!"></asp:RequiredFieldValidator></center> </td> </tr> < ...