First character of textbox should be alphabet and after that alphanumeric values in ASP.Net

Try this.

It allows A-Z or a-z in first alphabet while the rest can be either digits or alphabets

 <asp:TextBox ID="txtId" runat="server" />
    <asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server"  ControlToValidate="txtId" ErrorMessage="Required"></asp:RequiredFieldValidator>
    <asp:RegularExpressionValidator ID="RegularExpressionValidator1" ErrorMessage="First Character should be Alphabet."
        ControlToValidate="txtId" runat="server" ValidationExpression="^[A-Za-z][A-Za-z0-9]+$" />
    <asp:Button ID="Button1" Text="Validate" runat="server" />