This is all in the docs (rclone.org) but since I'm so nice I will handhold you a little 
--log-file=mylogfile.txt
(I think you can also just write that as --log-file mylogfile.txt if you prefer)
--log-level INFO
from least verbose to most: ERROR or NOTICE or INFO or DEBUG (default, NOTICE, is usually sufficient)
see documentaion for more info : https://rclone.org/docs/#log-level-level