Ringworlds are unstable. There is a net gravitational pull towards the near surface, meaning that a light source in the middle will eventually fall onto the ring (or, for something star-sized, the ring will fall into the star). You need an active stabilization system (magnetic manipulation, solar sails, attitude jets) to keep this from happening.
A non-spinning ringworld has a net gravitational pull towards the surface of the ring, but it's not very strong for anything a reasonable person would consider a "ring". In order for a practical civilization to live on the inside surface, you'll need to spin the ring at a pretty good speed to generate gravity through centrifugal force.
The material of the ring needs to be strong enough to hold itself together against the forces generated through spin gravity. Using the thin-wall formula for hoop stress (a reasonable assumption for something people would call a "ring"), a whole lot of things cancel out, leaving
stress = density * acceleration * radius
density is the density of the material the ring is made out of,
acceleration is the effective surface gravity due to spin, and
radius is the radius of the ring. Here's the bad news:
radius is huge. For a ring only a thousand kilometers in radius, you're looking at hoop stresses on the order of 10-100 gigapascals, requiring exotic materials such as carbon nanotubes to hold things together.
Your ringworld needs walls to keep the atmosphere from falling off the edges. The height of the atmosphere is described by a parameter called "scale height", which depends on the temperature and molecular composition of the atmosphere, and the effective surface gravity. As a rule of thumb, the walls should be at least ten scale heights tall; for an Earth-like environment, that's about 80 kilometers.
If you've got any sort of soil-like surface (as opposed to a purely rocky or technological surface), you need a way to counteract erosion. Water flow will tend to wash soil into the nearest body of water, and a ringworld doesn't have the geologic cycles to bring it back up through mountain-building.
A natural inhabitable ringworld isn't going to form. Even ignoring that there are no known processes that can form one, the material strength and orbital instability issues will keep it from forming. A high-tech civilization can build one, but the larger it is, the higher the required technology level.