How To Know If A Record Exists

Hello,

I have a TextBox and an Insert Button, it works like this:
 protected void Button1_Click(object sender, EventArgs e)
{
String strConn=SqlDataSource1.ConnectionString;
SqlCommand cmd = new SqlCommand("INSERT INTO My_Table_1 (Column_1) VALUES ("+TextBox1.Text+")", new SqlConnection(strConn));

cmd.Connection.Open();
cmd.ExecuteNonQuery();
cmd.Connection.Close();
} 
But,
what code i must write for checking, if the value i am trying to insert already exists ??

I mean something like this:

if (TextBox1.Text does not Exists on any Record in My_Table_1) then
        Insert it
else
        Show Message : "Already exists a record with this value"

Thank you SO MUCH, guys,


Carlos.