I'm trying to write a service for Trytond

There are tons of things i don't know/understand

But this is the first one:

The Tryton manual states that I need to indicate a "data directory" to the daemon and that such directory should be:

"The directory where Tryton stores files and so the user running trytond must have write access on this directory.
Default: The db folder under the user home directory running trytond."

See here (notice that this is in the "database" section of te configuration)

http://doc.tryton.org/4.4/trytond/doc/topics/configuration.html#database