uhf 50 ohm antenna
It depends on what you already know. You need to be familiar with electrical theory and RF basics. Then you need to have some understanding what is antenna and how it works. For this I can recommend you a book Antenna for all applications (if I remember the title right, but it should be very close). There you will find good explanation for antenna basics, what is antenna, the radiation resistance and radiation efficiency. There are also good chapters on different kinds of antennas. All this valuable information was written for general antennas, whereas RFID may require different conditions, namely near field when the distance between antenna and RFID tag is smaller than near field zone. Depending on application, antenna directivity may be very important for RFID. There is no one recipe for all situations.
You also may try to do search for RFID antennas and applications. There are some companies who make RFICs and even antennas for RFID. Many of them have application notes and other helpful information.