SQL Select Statement With A 'string' Variable?

I'm trying to add a 'change password' control to my site and seem to be having some issues.  I have code that works if I statically define what user is displayed on the form, but I cant get it to detect the 'authenticated' user and show them the reset for for that ID.

If I take the "+ myid" out of the select statement and just define the username statically the form works properly.  

 Error:
System.Data.SqlClient.SqlException: The column prefix
'System.Security.Principal' does not match with a table name or alias name used
in the query.
 
Here's a piece of the code that is supposed to detect the current logged in user.  However, it gives the error. (some of the code may be redundant but its not causing issues that I can tell)
 
 public void InitPage()
    {
            IPrincipal p = HttpContext.Current.User;
            String myid = HttpContext.Current.User.ToString();
            SqlServer sqlServer = new SqlServer(Util.SqlConnectionString());
            DataTable dt;
            SqlConnection cnn = new SqlConnection(ConfigurationManager.ConnectionStrings["myconnection"].ConnectionString);
            SqlDataAdapter cmd1 = new SqlDataAdapter("select * from USER WHERE USER_NAME = "+ myid, cnn);
            DataTable UIDtable = new DataTable();
            cmd1.Fill(UIDtable);
            User_Id.Value = UIDtable.Rows[0]["ID"].ToString();
            dt = sqlServer.USER_SELECT(Util.SiteURL(Request.QueryString["Pg"].ToString()), User_Id.Value);