The following script is from movie "Hitchcock"
A:What if it's another Vertigo?
B:You just get the first take under your belt, you'll be fine.
What does the bold part mean?
A take is an attempt to get a scene on film by actors. Usually it is not done correctly the first time so it is usual for there to be multiple "takes."
B is telling A that once he gets the first attempt out of the way, which won't be succesful, he'll be fine. He is implying that no one's first attempt is successful.
To have X under your belt is an idiom that means "to have experienced X for the first time."