cmd.Parameters.Add(new SqlParameter("@stationary", System.Data.SqlDbType.VarChar, 40, "Stationary"));
cmd.Parameters["@stationary"].Value = textBox.Text;

cmd.Parameters.Add(new SqlParameter("@stationary", System.Data.SqlDbType.VarChar, 40, "Stationary"));
cmd.Parameters["@stationary"].Value = ??; //OR this whole row is wrong - it has to be bind to the "Stationary" Column somehow.

If (checkbox1.checked == true)
{
// If the check box is ticked do some code here
}
else If (checkbox1.checked == false)
{
// If the check box is unticked do something here
}

SELECT CompanyName, StationaryNumber
FROM Company
WHERE StationaryNumber LIKE @stationary + '%'

cmd.Parameters.Add(new SqlParameter("@stationary", System.Data.SqlDbType.VarChar, 40, "Stationary"));
cmd.Parameters["@stationary"].Value = checkBox1.Text;

listBox1.MultiColumn = true;

// do a loop for your sql statement...assuming you are using a SqlDataReader
SqlDataReader dr = cmd.ExecuteReader();
while(dr.Read())
{
     string data = string.Format("{0}\t\t{1}", dr.GetString(0), dr.GetString(1));
     listBox1.Items.AddRange(new object[] {data});
}

string telNumbers = "SELECT CompanyName FROM Company
WHERE StationaryNumber LIKE @stationary + '%'"
SqlCommand cmd = new SqlCommand(telNumbers, sqlConn);
sqlConn.Open();  
SqlDataReader reader1 = cmd.ExecuteReader();
while (reader1.Read())
{
      checkBoxStationary.Text = reader1.GetString(0);
}
reader1.Close();
sqlConn.Close();

while (reader1.Read())

while (reader1.ReadLine())

checkBoxStationary.Text = reader1.GetString(0);

checkBoxStationary.Text = reader1.GetString(0, 1);

private void ButtonIskanje_Click(object sender, EventArgs e)
                {
                    string companySearch = "SELECT ImePodjetja FROM PodjetjeWHERE " +
                                           "((@pošta IS NULL) OR (Podjetje.Pošta = @pošta)) AND " +
                                           "((@kraj IS NULL) OR (Podjetje.Kraj = @kraj)) AND " +
                                           "((@stacionarni IS NULL) OR (Podjetje.Stacionarni = @stacionarni))";
                    //POST NUMBER (POŠTA)
                    if (textBoxPošta.Text != string.Empty)
                    {
                        cmd.Parameters.Add(new SqlParameter("@pošta", System.Data.SqlDbType.Int));
                        cmd.Parameters["@pošta"].Value = textBoxPošta.Text;
                    }
                    else
                    {
                        cmd.Parameters.Add(new SqlParameter("@pošta", System.Data.SqlDbType.Int));
                        cmd.Parameters["@pošta"].Value = String.IsNullOrEmpty(textBoxPošta.Text) ? DBNull.Value : (object)textBoxPošta.Text;
                    }

                    //CITY (KRAJ)
                    if (textBoxKraj.Text != string.Empty)
                    {
                        cmd.Parameters.Add(new SqlParameter("@kraj", System.Data.SqlDbType.VarChar, 30, "Kraj"));
                        cmd.Parameters["@kraj"].Value = textBoxKraj.Text;
                    }
                    else
                    {
                        cmd.Parameters.Add(new SqlParameter("@kraj", System.Data.SqlDbType.VarChar, 30, "Kraj"));
                        cmd.Parameters["@kraj"].Value = String.IsNullOrEmpty(textBoxKraj.Text) ? DBNull.Value : (object)textBoxKraj.Text;
                    }

                    //CHECKBOX STATIONARY NUMBER (STACIONARNE ŠTEVILKE):
                    if (checkBoxStacionarni.Checked)
                    {
                        cmd.Parameters.Add(new SqlParameter("@stacionarni", System.Data.SqlDbType.VarChar, 40, "Stacionarni"));
                        cmd.Parameters["@stacionarni"].Value = "Stacionarni"; //checkBoxStacionarni.Text;
                    }
                    else
                    {
                        cmd.Parameters.Add(new SqlParameter("@stacionarni", System.Data.SqlDbType.VarChar, 40, "Stacionarni"));
                        cmd.Parameters["@stacionarni"].Value = DBNull.Value;
                    }

                    povezava.Open();
                    SqlDataReader bralec = cmd.ExecuteReader();
                    while (bralec.Read())
                    {
                        listBoxListaPodjetij.Items.Add(bralec["ImePodjetja"]);
                    }
                    bralec.Close();
                    povezava.Close();
                }

private void checkBoxStacionarni_CheckedChanged(object sender, EventArgs e)
        {
            listBoxListaPodjetij.Items.Clear();
            string stacionarneŠtevilke = "SELECT ImePodjetja FROM Podjetje WHERE Podjetje.Stacionarni LIKE '%'";
            SqlCommand cmd = new SqlCommand(stacionarneŠtevilke, povezava);
            povezava.Open();  
            SqlDataReader bralec = cmd.ExecuteReader();
            while (bralec.Read())
            {
                //WHAT COMES HERE!!  In my opinion I need all the companynames which have stationary number! but how to save it?
                //and then to call them in event button_Iskanje (Search) where checkBox is checked!
            }
            bralec.Close();
            povezava.Close();
        }    

SELECT CompanyName
FROM Company
WHERE ((Company.Stationary IS NOT NULL) AND (Company.Stationary LIKE '01%'))

SELECT CompanyName
FROM Company
WHERE Company.Stationary LIKE @stationary + '%'