The Navier Stokes are the three "laws" of physics, conservation of mass, momentum (in all directions) and energy. This gives 5 coupled, partial differential equations.
They are very important to know inside and out before starting CFD because you will NEVER get the correct answer in CFD. you need a solid understanding before attempting to resolve any solutions.
Having said that, the distances associated with viscous flow are order(s) of magnitude smaller than the mean flow. So, getting good viscous results are kind of a pain in the ***. In fact, more often than not, we find ourselves solving the Euler equations rather than the full N-S which do not have viscous effects.
Eithre way, before diving into any software-specific tutorials, get a good background. Check out a little write-up I have for some decent background specifically on the derivation of the NS equations.
http://www.eng.utoledo.edu/~mheminge/CFD_Notes.pdf