Peckish

I agree with Tartan, a straight edge and a curve are good places to start. The problem with longarm rulers is they can be fairly expensive and nobody carries them in retail stores, so you pretty much have to order them directly from the designer/maker. And the kind of rulers you will find useful depend mainly on what kind of quilting you want to do, so it's a very personal decision.

Lisa Calle, Bethanne Nemesh, Deloa Jones, Jamie Wallen, Margaret Solomon Gunn (I'm sure there are more, but these are the ones I have purchased from) are all professional quilters and have designed specialty longarm quilting rulers. I think the first thing you should do is find videos on their websites, YouTube, and/or Facebook, and watch them demonstrate how they use their rulers. Ask around locally to see if you can borrow a ruler or two from a quilting friend. I was actually able to borrow a couple of rulers from my local Innova dealer and test them out.