I've been into cycling for decades—riding with grade-school buddies, kicking around in college, then city commuting in Boston, Paris, Barcelona, New York, and now Seattle. Somehow only in the last 10 years, when I became a volunteer mechanic at a bike-centric Seattle nonprofit, did I think about adjusting anything more than the seat height. Now I’m diligent about making sure I have the right bike and that it’s fit for my body and my riding style.

I wish I could have figured that stuff out sooner, so for this story, I wanted to gather expert advice on the basics of what people should look for and what questions to ask when buying a bike. (My colleague Michael Venutolo-Mantovani has more advice if you’re going that route.) After that, I’d take my own bike to a professional fitter and go deep into the nitty-gritty of bike fit to find out what it means to have a tailored ride.

I start with no bike at all.…