...I'm just going to assume you're using the legal free student versions of 3ds max and maya and have purchased ZBrush as there are no free student versions available for that program.
Now what exactly is your problem? If you don't know how the programs work, I don't think anyone will be able to teach you through forum posts, as they are very, very complex tools. You should probably just search for tutorials and work your way from beginner tuts to more advanced ones until you know enough to model such a model.
If its just a workflow thing, I would first search for human references to ensure that I get the anatomy right. There are a lot of good references in the web, just search for a male model that has roughly the proportions you need. This model will be modeled in 3ds max or maya and have a neutral V-pose (I mean a T-pose with more relaxed arms, better for skinning later
) It will serve as a base mesh for sculpting. I would then achieve the bend over pose in Mudbox (Or Z-Brush in your case) using the posing tools there.
Then come the metal parts (modeled in max or maya too), and next step would be sculpting the wounds, muscle details, maybe some part of the rusted metal, until I have a highpoly I'm satisfied with.
I would also use the paint texture tools in mudbox (I think Z-Brush has something similar, although it's only vertex color i think) to create a basic (and most importantly - seamless!) texture. If I have a mudbox version available that allows for a P-tex setup I'll use that, If I don't, my highpoly will need to have UV coordinates (created BEFORE exporting the basemeshes to Mudbox of course
)
Now I'd create a lowpoly, probably by exporting the lowest subdivision level of my mudbox file and changing/ optimizing it until it has a reasonable polycount and a topology that allows for good deformation. This will then also get a proper UV unwrap, with optimized usage of the UV space (mirrored /overlapping uvs, more space for important areas like the face etc.).
After this is done, I'll use my highpoly to bake a normal map, an ambient occlusion map and my base texture onto my lowpoly. Depending on polycount this can be done in 3ds max, maya or in a seperate tool like X Normal. Baking a normal map in Mudbox is possible, but not recommended as you need smoothing groups to achieve the best result, which mudbox doesn't support. (if using P-Tex, the base texture will have to be baked there, though)
Theeeen, if everything baked properly (which it never does on first try -.-), I'd open photoshop and refine my diffuse texture (colors), create a specular and, if needed, gloss map and maybe use a plugin like nDo or the Nvidia normal map plugin to add some more little details to the normal map I didn't feel like sculpting or forgot. The baked ambient occlusion is simply multiplied over the colors as a layer in photoshop.
Sooo...after giving you a short overview over this, quite frankly, nightmarishly long process, what exactly would you like to know in detail?
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I need help for modeling a monster
I need help for modeling a monster 
