Devil Trigger, by default, is R3/L3, I think... Many people change it, because it's a pain. Not sure which mission you unlock it on, but you can find more info on changing the buttons below: http://pcgamingwiki.com/wiki/DmC:_Devil_May_Cry#Modify_the_config_file
Everything else can be changed easily. I think most people put the weapon switching on one side, and both dodges on the other, but it's really up to personal preference.