To learn how to operate large commercial trucks, drive on city streets and highways, and finally to take CDL test, you can go through available training programs.
These courses include instructions about the regulations long-haul trucking and behind the wheel training on loaded and unloaded trailers.
Especially useful thing you will get is pairing with experienced drivers as part of completing on the job training,
Conclusion
OTR drivers are having truly exciting life. Still, the truckers lifestyle, or should I say perception of their lifestyle, is the main reason why small number of young people chose this profession.
If you are considering being an OTR truck driver, the best thing would be going on a run with the driver to get the idea of the job. If not, do some research. There are plenty of useful sources on this matter.