Check out two sources
The first has a vast video library with great info on digital painting. You will learn the process and the tools.
The second has great info on human anatomy and drawing portraits and figures.
But, the main problem is: you wont get good enough without unreasonable amouts of tiem and hard work
If you have a job or university to go to, AND you are making your own games, you already have enough work on your hands. Learning to make art is hard work and lots of it. The sites I recommended often tell you that you need to be patient, that you need to draw everyday for 1-2 hours at the minimum, and you will have to work for months for substantial improvement.
Im playing around with varous forms of art as a side hobby, and after a year or so, my work is nowhere near worth putting into a game.
The problem with learning art, from a programmers point of view, is that you cant understand it just by reading and seeing it done. If you see some code, have it explained to you, and you can reference it while making your own attempt at the subject, you can get good at it very fast and produce reasonable results quickly. In art, you can intelectually understand the process, but it is not enough to go through it yourself. You have to teach you brain a subconcious understanding of form and lighting, and you have to acquire needed control over what you draw. This can be only done through work work work.
Working in 3D is the same, just harder, because drawing in 2D is almost a prerequisite for 3D work.
And even if you could produce a piece of acceptable quality having little training and experience, it wont be worth the time investment. In the time you make the piece you could write code for someone to earn enough cash to hire an artist.
Learning art is fun, but only if its a goal itself. You see it as a means of acquiring art for your games, "you're gonna have a bad time".