Knowing what to buy in O-gauge for a beginner unfortunately takes a fair amount of study and practice. After spending my whole life totally ignoring O-gauge/O-scale, I first started getting into 3-rail O-gauge about 10 years ago. I've learned quite a bit in the last ten years, but still don't know everything, unfortunately. I will say one thing, a great place to learn about various sizes is to shop for used stuff at train meets. I have learned a lot and scored some really good deals that way. And do some research on the internet. There are several various sites and blogs that discuss the different sizes and show examples.
Speaking of still not knowing everything, I recently ordered three box cars from an internet supplier. One was true 1/48 scale, and I knew that upon ordering it. As for the other two, I thought one was traditional-sized (approx. 1/56 scale), and the other was O27-sized (approx. 1/64 scale). As it turned out, after receiving the order, the other two BOTH ended up being O27-sized. Not a big deal, I kept all three because I liked them anyway. It just goes to show, that even though I'm semi-retired now, you're never too old to learn something.