Problem rigging weight paint I'm new here and a beginner of blender.
i have a problem with rigging, some parts of my mesh deform. i'm italian so sorry if i speak very badly.
that's the problem:




the back is deforming when i rotate the on the x axis the leg. 
the model is only one mesh, i don't know if that can be a problem, the piece of armor on the leg is connected to the back.

sorry for the question or my writing, i'm following a lot of tutorials but i cannot resolve this.
Thanks in advance
So, here is my opinion:


First you need to put the Armature modifier above the Subsurf modifier, and enable the Preserve Volume option.
You should add a little more weight on the butt for the leg, you'll see that it works better when you rotate the leg
Maybe add a hip bone and weight paint to add a little bit more precision in the mesh control?