ok, if you know box modeling it will be a lot easier, since all base mashes you will have to make in diffrent soft.
There are 2 ways to go.
-
you want to make high poly sculpts for normal maps and low poly baking only. In this case any sculpting softewere will do. From modo, 3d max, to blender. You have to go to the site of you softwere and download a tutorial, since the way to do it will be diffrent depending on what you use.
-
You want to make high poly sculpt for more artistic purpose, displacment render, and really good normal maps.
The only way to go here is to either download zbrush or mudbox. Its hard to say witch is better, im using zbrush, but mudbox is simplier and a lot more efficient, but zbeush can handle more polys. zbrush is 600$ and mudbox is 800$, but i would wait with buing one until zbrush 4 comes out. Its not cheap, but if you are serious about 3d, you will have to get one eventiually.
You can try both, and check out witch one works better for you. here are free 30 day trial versions. its enough to learn a lot.
https://www.pixologic.com/zbrush/trial/
https://usa.autodesk.com/adsk/servlet/mform?siteID=123112&id=10707843
and scullpt… or if you want to do it the right way, and save yourself and lot of time …
then first thing you need to do is buy a good anatomy book, and momorise all main human muscles and bones, their shape and placment, you dont need to know the name. 90% of the sculpts even alien and monsters will be based on homan anatomy to look natural.
Then register on zbrushcentral.com, you will find tehre tutorials, references and all that shit.
For zbrush and mudbox download the official documentattion and fallow the main tutorial that comes with them, will teach you the basics of using the soft.
After completeing the main tutorial witch schould take you few days, write how its going, then i can tell you how to skip a few months/years of learning.
