The skin part of him, at least the arm part, looks like a slightly tapered cylinder with a small bend in it.
OK, I took a look at your blend file. You need a front and side view of your model. You should draw them side by side, so they match up when you use them as background images when you begin to model. Something like this, except with the armor and the proportions of your cyborg man. (Notice in the referenced drawing, key locations such as the chin, the knees, the bottom of the ribs, the ankles, etc, line up between the front and side drawings.)
Then take a look at a good character modelingtutorial. The link is to an oldie but goodie by Jonathan Williamson, and is about 70 minutes long. It covers the process you’ll be going through in a lot more detail than can be done in a simple post here on BA.
If you run into trouble or have questions, people here will be glad to help. Welcome to BlenderArtists