Welcome!
Looks like you're starting like so many of us did: "I want to be a Stormtrooper. Now what?"
To get it right, you'll be building the suit. The ready-to-wear "costumes" just are not the same quality, accuracy,etc. and really don't save you that much. Decide WHICH Stormtrooper you want to be. Old school original trilogy? Clone? First Order?
Also, let us know where you're based so we can help with local suppliers and you'll get answers from others in your area.
This community is FULL of helpful people ready to answer any and all questions for you. We've all been down this path.
It can be overwhelming, so ask LOTS of questions and we'll be glad to help you on your journey.