Something

using System.Data.SqlClient;  // BUNU SAYFANIN EN USTUNE YAZICAKSIN

private string Conn = ConfigurationManager.ConnectionStrings["Baglanti"].ConnectionString; 

string isim = TextBox1.Text;
string mesaj = Textbox2.Text;

SqlConnection baglanti = new SqlConnection(Conn);
string sorguekle = "INSERT INTO icerik(baslik,katid,mesaj) VALUES(@baslik,@katid,@mesaj)";
SqlCommand command = new SqlCommand(sorguekle, baglanti); 
command.Parameters.AddWithValue("@baslik", isim);
command.Parameters.AddWithValue("@katid", 1);
command.Parameters.AddWithValue("@mesaj", mesaj); 
baglanti.Open();
command.ExecuteNonQuery();
baglanti.Close();