Rest 60 to 90 seconds between sets on accessories and 2 to 3 minutes on heavy compounds. Stop a set 1 to 3 reps before form breaks down. Pain that is sharp or joint-related means stop and get it checked.
Pick an exercise from the list or type your own if it is not there. You can also tap "+ Add" on exercises in the Train tab. New exercises go into the day marked "Adding here".