how to retrieve data from database using the combo box?

Try
            strSql = "select col1,cl2 from table1"
'execute sql and save it in tbk1 datatabel
             tbl1 = objDb.getTable(strSql)
'add default select column
            dim drw as datarow
            drw = tbl1.NewRow
            drw.Item("col1") = "Select"

            tbl1.Rows.Add(drw)
            combo1.DataSource = tbl1
            combo1.DisplayMember = tbl1.Columns(0).Caption
            combo1.ValueMember = tbl1.Columns(1).Caption
            combo1.SelectedIndex = -1
            combo1.Text = "Select"

<pre lang="sql">
dim strSql as string =select * from table 2
         where   
'some condition
              If combo1.Text.ToLower.Trim <> "select" Then
            strSql &= and  table2.column= combo1.SelectedValue 
        End If
</pre>