okey dokey this is like the 2nd tutorial Ive ever done so bear with me, I've recently been sketching a lot of Terminator armor so while it's still fresh in my head that's what I planned for this pic, the brushes I used can be found here http://adonihs.deviantart.com/art/My-Brush-Pack-118954791 I'll tell you the brushes I used throughout but it's better to experiment with them to find out which ones you're more comfortable with...

step.1 I started out with an A4 canvas at 100dpi. using the standard painting brush I blocked in some really basic shapes, I used a brown gray because the color will come out more later on when I'm experimenting.

step.2 using a much lighter gray/brown I then add a light source and work out a little bit of the detail, keep it loose!!!

step.3 start on a new layer and using the detail brush add some more....uh....detail also get some mid tones into the picture to solidify the character, I done a little bit in the background too no biggie I'm not gonna do very much with it

step.4 duplicate the upper layer and set the blend type from normal to overlay, this will help it 'read' a bit better and bring out some of the colors, of course I could have made the pic darker from the start, this is just a less time consuming alternative and I like to work as fast as I can to keep the original idea as vivid in my head as possible.

step.5 this is my favorite part because its like modifying your own space marine character, now I start to make the bigger design decisions, if you already have an idea what chapter your going to make his this is a good place to think about what details that chapter has though at this point I had no idea so I just carried on drawing a terminator.

step.6 increas the image size to 300dpi at this point I decided to mess around with the position and size of some of the stuff in the pic, easy as pie to do- press L for the lasso and draw around any part you want to change then press Ctrl+T and you can transform tour objects scale or rotation also it you right click in the transform square you will have another list of tools I recommend 'warp' its awesome for messing around and moving things. Then I added textures mwahahahahahahaha, there are hundreds of textures available on the net find some dirty, scratchy looking ones and lay them down on a new layer, I used a couple of different textures here and while not being anal about it kept them mostly on the meta parts of the armor

step.7 I changed the layer type to 'vivid light' but you might like the look of another layer type, experiment!!!!!!!!

step.8 right about now you entire palette is on the paper, just press Alt to color pick it's so much easier than fumbling around with the color slide, anyhow I generally use a really light gray detail brush to draw in the detail and then use a much bigger dark gray brush to shade over it (though its the same brush, right click to bring up the diameter slider) and I keep repeating this step til I got the level of detail I want for the pic, this is where the work is folks it can take half an hour like this pic or a couple of days if want to cram the detail in

step.9 finally decide what chapter the terminator is going to be and add final bits and bobs accordingly, nothing that special here
space wolves logo- on another layer draw the emblem and colors ans set the layer to color burn(also try multiply and overlay), mess around with the opacity until you got the look you want
eye lighting-on a new layer draw some spiky lines coming out of the eyes then go to effects-blur-radial blur-zoom and place the zoom center as close as possible to the middle of the forehead, mess around with the opacity blah blah blah........
.....and your done!!!!!