Hi,

ComboBox.DataSource = objSqlDataset;
ComboBox.DataTextField = "DisplayCol" // Column name which you want to display.
ComboBox.DataValueField =  "ValueCol"; // Column which you want to use for value.
ComboBox.DataBind();

Steps:
1. Drag and drop the combo box
2. If you see the dropdownlist in design mode, you can see the small greater than symbol, If you click on it you have a option like choose Datasource,
3. Then you can connect to your database.