Nice slides by Christian Terboven on “OpenMP in the Multicore Era”. Enough to get started.
By the way, to build an OpenMP program with modern GCC it’s enough to
gcc -fopenmp -o eval42 eval42.c
It should work out of the box. GCC 4.1 supports OpenMP 2.5, and GCC 4.4 supports OpenMP 3.0.