Depending on your hotkey setup, there's a key to enable your jetpack which is I THINK double jump but I might be mistaken. All you really have to worry about is running out of your jet meter while in the air, because fall damage in this game is ridiculous. If you bunny hop up along beams and stuff, you can reach almost anything in that picture.

Same with cliffs. Always leave yourself some jet pack bar to work with, and land on a ledge or something similar as you're "climbing" the air. It's a really easy way to take shortcuts, and avoid enemies you're not quite prepared to deal with.

Hope this helps! The jetpack is quite literally a life saver, so start practicing using it as much as possible. The bar auto refills when standing on a ledge or surface, as long as you're not in the air, so use it liberally. Be prepared to die a lot while learning how to use it.

-Cym