Thats a broad question...
The general process of comp'ing 3D in 2D requires a model in a 3D software package and 2D photo-manipulation software.
The most common setup is 3D Studio Max and Photoshop, though the level of software is entirely guided by your budget. Sketchup, Lightwave, Modo etc are all capable 3D softwares with varying levels of depth.
For the comp'ing process, you'll need to read up on texturing, camera matching, direct light and GI solutions, the rendering process, colour correcting and masking.
There are some widely applauded books for varying levels available at
www.3dats.com i believe.
Anything more specific you wanted to know?