Passing a polygraph to become an officer?

i am currently a student earning my bachelors in social and criminal justice with a specialization in forensics, and i wand to become a CSI. but before i can be a CSI i have to be a police officer for many years. not something i absolutely wanted to do i am a whole 5 feet and 2 inches tall lol. i am currently in my 20th week of pregnancy so i am not looking to join any time soon. a requirement is to take a polygraph test, and i know i will not lie but i also know i will be nervous, will nerves misinterpretation my results? from what i understand they will be asking questions about drug usage and criminal history. i have honesty beat a few addictions and although i will be truthful, being honest about things like that naturally will raise my pulse. its not something i am proud of at all. how can i stay calm when i have to be honest about difficult things? i dont want them to think i am lying about anything.