Usually you want .onLoad, which—as the name suggests—runs when the package is loaded. If something has to happen before anything is run, that's the way to go. onAttach only runs when the library is attached, e.g. when somebody calls library(your_package). onLoad will also run when somebody loads but doesn't attach your package by calling your_package::your_function.
Reading:
http://r-pkgs.had.co.nz/r.html#r-differences
http://r-pkgs.had.co.nz/namespace.html#namespace
and the official docs at ?ns-hooks, if you haven't already read that.