Retopology for human characters

To learn it? One of the best free resources for that is going to be Youtube. If you have some time, do a Youtube search for retopology in whatever software you're using. There are going to be hundreds of results. You just gotta have time to sort through them and pick up the tips.

There are algorithms and addons out there, depending on the software you are using, which could do a quick retopology automatically, but the results are never as good as if you do it by hand and get exactly the edge flow and poly count you want. However, this requires knowledge about topology. Modeling experience. You need to know not only the features of your tool but also the best way to apply the polygons so that they can deform if needed or be UV unwrapped if needed, etc. It's something that will take time to learn properly because every object you retopo will require slightly different conditions.

What software do you use for modeling or sculpting? Start there by searching for retopology tutorials.

Let me try posting that link again. The formatting was horrible in that last reply.

https://www.youtube.com/watch?v=CuQzPDs99yM

One thing to keep in mind is that you should try to keep everything quads (triangles are ok in some areas, but never use ngons), and try to keep the edge loops flowing consistently with the curves of the sculpt. This will help with deformation if the model is going to be rigged or deformed in any way. Quads also help with UV unwrapping, as you may already know.

There are some addons for Blender that might help you out once you understand how topology should flow. My favorite is a product called Retopoflow. Blender can do retopology without addons, but Retopoflow makes things a lot easier. There's also another one called Quadwrap, which is useful for limbs and fingers and cylinder shapes like that. Lastly, there's an automatic retopo addon called QuadRemesher, which is surprisingly good for certain objects and can do clean, quad topology in one click. It's a superior algorithm to InstantMeshes in my opinion.

Good luck!

Just to point out one thing regarding this. If you want to retopo models that are scanned by you that's ok, but if you plan to retopo other peoples models (with this i mean, other peoples work, charachters ripped from games etc.) that can and will get you into big trouble. Because as naive this question is it isn't because now everyone who have even slightest amount of 3d knowladge can use those tutorials and advices do fast retopo of models that will end up on sale here and other marketplaces. In my opinion this kind of discussion should be talked in private.