You are going to need to execute a reader, read the query results, and save the results into your property. I would look something like this:
SqlDataReader rdr = cmd.ExecuteReader();
while (rdr.Read())
{
txtsupplierno.Text =rdr["SupplierId"].ToString();
}
This would loop through each record and put that value into the text field, overwriting the last value. It isn't ideal if you are going to return more than one row but it gets the job done (quick and dirty first pass).
Next you need to think about protecting yourself against SQL injection. You should never pass a value from the UI straight into a SQL statement. The user could put code in the text box that would give them control over your database. If your combobox does not allow values to be typed in, you are safer, but I still like to see values to be inserted into statements using parameters.
Here is a good tutorial with more information that should help you out:
http://www.csharp-station.com/Tutorial/AdoDotNet/lesson03[
^]