also tried to use the code below (and others code) but i got errors:
You seem to have a fundamental misunderstanding of serial communications.
if (RFID.available()>0)
{
Serial.print("Card number is:");
Serial.print(RFID.read(), HEX);
Serial.println(" ");
}
It is likely that the RFID will return several characters for each card scanned, not just one. You need to collect data in an array until the end of packet marker arrives. What is that end of packet marker? Beats me. You're the one with the no-link-provided RFID reader.
The first code that you posted should allow you to determine whether there is a common end-of-packet marker or a fixed tag size. Either way, you need to collect the characters that make up the packet into an array, NULL terminating it after each character addition, and then used strcmp() to see if the tag read matches a (or one of a list of) predefined tag strings.
As for the errors uploading, it appears that the RFID reader interferes with sketch loading, though it shouldn't.
Post a link to the reader/writer.