Pipe Threaded pipe needs to have a sealant applied to the threads in order to seal the joint.
Many different sealants are available with many different formulations.
In general, most sealers are spread over the male threads but you should check the container. A wire brush is the perfect tool for this. Fill all of the valleys between the threads. If you are using Teflon tape or other tape product, wind it around the threads and break it off. Smooth the loose end down onto the threads.