In Web.config:
<appSettings>
<add key="defaultCheckBox" value="CPP"/>
</appSettings>
In aspx:
<asp:CheckBoxList ID="CheckBoxList1" runat="server">
<asp:ListItem>Java</asp:ListItem>
<asp:ListItem>c#</asp:ListItem>
<asp:ListItem>CPP</asp:ListItem>
<asp:ListItem>SQL</asp:ListItem>
</asp:CheckBoxList>
In aspx.cs file:
string defaulCheckBox = System.Configuration.ConfigurationManager.AppSettings["defaultCheckBox"];
CheckBoxList1.SelectedValue = defaulCheckBox;