Well. Sure you do need to use pretty much your whole body and its muscles in parkour but in my opinion legs and back are mainly the most important ones... If you think about it, what's the big deal of having great shoulders and triceps if you're not even able to run one hundred metres after landing.
They say that that first step for beginners is to practice landing and therefore 'roll' which is often performed after landing a drop 'cause it spreads the pressure against legs and back after (high) drop to a whole body. After "mastering" roll and landing it should be more safer contiuning to upper levels.
A monkey vault, a (king) kong vault, and a speed vault are pretty safety ones to start practicing parkour with.
There exist dozens of advice on the internet to follow and ones from prof' traceurs are for sure much better than my ones but I think it's good and safe to start with my advices as well
don't be afraid to ask me more about parkour either via this topic, pm or vm. Just have fun with parkour that's what it's all about like other hobbies
