Hello,
I am trying to lear bro from scratch. i don’t know where to start.
Hello Cesar,
you have already started by using this mailing list 
This is a first
contact point for every question that you can't get answered through
documentation.
Depending on how you learn you might find the right type of tool on the
Bro documentation page.
https://www.bro.org/documentation/index.html
There you can find classic how-tos, exercises, Videos, and other
sources. Whenever you get stuck with a topic please come back to this
mailing list!
Welcome to the Bro world!
Doris Schioberg