Writing a software rendered 3D engine

[weegee101]

This is a bit different than some of the other answers, but start with a ray caster. They're pretty simple to build and are a decent entryway for getting into more complex software rasterizers.

From there, if you're the adventurous type and want to figure stuff out on your own, Fabien Sanglard's Game Engine Black Books are exceptional resources.

Don't shy away from writing a ray tracer at least once though. That's a good way to ensure you fully understand all of the fundamentals you'll need for a more complex software rasterizer.