You will need to measure the threads that you are wanting to duplicate ( or mate to ).
Measure the outside diameter ( O.D. ) of male threads with a vernier caliper, and measure the thread pitch with a thread gauge ( which is a collection of metal pieces each having a different serrated or notched pattern that you hold up to your threaded piece and, by trial and error, determine the right match for your threaded piece). . This process will tell you the die specifications. . I think you will get the idea fairly quickly if you go to a tool store and handle some tools, taps, and dies, and talk to someone there.
For female threads, the process is more tricky because you will be measuring (with the caliper) an inside diameter ( I.D. ), and to that measurement you will have to add twice the thread depth, ( after you have determined the thread pitch in the aforementioned manner), in order to get the required tap diameter.
That's the general gist of it, and hopefully someone else will elaborate with more precisely expressed techniques.
Ask at a tool store to see a thread gauge and you will soon get the idea of how to use it.