One of the best places to learn about trace formula, other than David Whitehouse's wonderful notes, are the notes by Erez Lapid. 

https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.494.5118&rep=rep1&type=pdf

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 are reasonable beginning. 
http://homepages.math.uic.edu/~rtakloo/papers/borel/borel3.pdf

My advisor Paul Garrett's book is, of course, my favorite place. They treat several examples, but do not do any higher rank case completely. 
https://www-users.cse.umn.edu/~garrett/m/v/current_version.pdf