How do I use parameterized statement? (no replies)

I have a method that accepts data and and is being called approx every second. I tried creating a parameterized query which inserts the results into my database but I keep getting error messages telling me that the parameter has already been defined. Can someone please explain how this works and what I'm doing wrong? I've read the mysql doc on using prepared statements and it doesn't go in depth enough on using prepared statements.

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private void (string _fname, string _lname, string _age)
{
string sql = "Insert Into myTbl (firstname,lastname,age) Values(@fname,@lname,@age)";
cmd.CommandText = sql;
cmd.PrePare();
cmd.Parameters.AddWithValue("@fname", _fname);
cmd.Parameters.AddWithValue("@lname", _lname);
cmd.Parameters.AddWithValue("@age" , _age );

cmd.ExecuteNonQuery();
}