Suggestion - Poster Designing

Hello there, first of all thanks for giving your time to read.
I am a film editing student learning from film school and PV club school too. I want to invest my skill towards film poster designing and all. I just know the fundamental of photoshop. Simply I say "to open and close" but started to learn photoshop. I don't able to figure out my need. Specially where to start. Few doctors also prescribed to learn illustrator too.. Please doctor, if you have any prescription. I highly appreciate.. If not, don't worry. Keep smiling, Stress free life all beautiful wishes to you.. Enjoy the day