You want to fetch data from database but did not define any commands.
After the below line
sqlCommand = sqlConnection.CreateCommand();
Add another line like
sqlCommand.CommandText = "Select ......";
(Write Select Query to fetch the data for gridview...)
Then modify the line
SqlDataAdapter sda = new SqlDataAdapter(sqlCommand.CommandText, connectionString);
to
SqlDataAdapter sda = new SqlDataAdapter(sqlCommand);
Add the line to bind the datatable to gridviw like
dataGridView1.DataBind();