it could be worth looking at the vellum solvers (hair, cloth, softbody) to start with, as SideFX have wrapped them up pretty well so you don't need to write any code to get started. Plus they're really fast and fun to play around with and there's also plenty of tutorials for them. The pyro solver is also wrapped up pretty nicely as well, and you can run the minimal solver on the GPU to see what various setting do.
In terms of character stuff, you can bring in characters via fbx (or usd skel), which brings in the skin, rest and animated skeleton and then edit the motion, add secondary motion or physics and do simulations, before exporting the baked mesh out for rendering. Since joints are just points with transforms in Houdini you can treat them as you would any other geometry; collide them with object, add lag or jiggle etc
In terms of rigs, there currently isn't anything equivalent of Skeleton 5 and atm, there are the low level tools for building that kind of thing, but the high level rig controls aren't fully fleshed out. Hopefully in the next version of Houdini, but for now would really use it for motion editing and transfer and secondary animation rather than full rigs.
This tutorial goes over where things are at. Also has example files, which is always nice.