Very nice work, especially for a first attempt.
I like the second one a lot (after you removed the blotch) - the unusual angle and lighting with the bright red color work very well for me.
On the third one I think the tattoo is slightly blurred, probably out of focus. Because it's obviously such an important part of the personality you capturing, I think it would help a lot if it was completely sharp - just stop down the lens a bit more (or by putting the focus in between the face and arm you might have had enough depth-of-field to have both in focus).
The shutter speed is also on the lower end for these things (people do move, and a tensed-up muscle shakes...), so you may have motion blur. You might have gotten a better tradeoff between noise and shutter speed at ISO 400.
I'd suggest you don't read up too much on portraiture - reading a bit is very useful, but too much will more likely make your portraits derivative, unoriginal and uninteresting. Keep shooting and develop your style.