One of the best places to learn about trace formula, other than David Whitehouse's wonderful notes, are the notes by Erez Lapid Introductory notes on the trace formula.
Arthur's trace formula relies on Langlands's work on Eisenstein series and Spectral decomposition from 1962–64. This stuff is extremely difficult for general reductive groups.
It would be near insanity to directly go to Arthur's papers or notes without mastering the spectral decomposition of automorphic forms on GL(2) and the Selberg trace formula. It is not wise to learn trace formula without learning about spectral decomposition.
For the general case, Borel's notes Automorphic forms on reductive groups are a reasonable beginning.
My advisor Paul Garrett's book Modern analysis of automorphic forms by example is, of course, my favorite place. They treat several examples, but do not do any higher rank case completely.
