Please help ! I have the kilt, I have the shirt, I have the hose, I have the shoes, I have the belt.. what I don’t have is the jacket!

Now, I know where to get them, what confuses me, is all the different types of ways the different places asks to measure you. I have never gotten a tailored anything. I have never even really worn a formal jacket ( I’m 23) The only thing I know for sure is my waist size and shoe size.

Some sites I’ve seen list there sizes in S M L XL, and so on. Others ask for a static number, what this number refers to I don’t know. Others ask for a slew of measurements.

Would it be possible for someone with more experience in this to give me a simple way to find what I need to know when buying/renting* a jacket? I would hate to spend a lot of money on something I want, but get it in a size to small or too large.

Thanks

* I have searched locally, and I do not think I have found a place in Portland Maine, or near by where I could rent a PC or Argyle.