click on combobox then in properties dialogBox set 'DropDown Style' to 'DropDownList'
for array, you need
string select= "SELECT column FROM table WHERE condition";
SqlConnection con = new SqlConnection();
con.Open();
SqlCommand cmd = new SqlCommand(selectcmd, con);
SqlDataAdapter sda = new SqlDataAdapter(cmd);
DataTable dt = new DataTable();
sda.Fill(dt);
con.Close();
combobox.DisplayMember = "column";
combobox.ValueMember = "column";
combobox.DataSource = dt;