Hello Philippe,
One of the more important elements of your Data Layer is the Universal Data Object. If you're looking for best practices on how to format your Universal Data Object (UDO), check out our Universal Data Object Guide (
https://community.tealiumiq.com/posts/611172). You'll should find some good guidelines there.
As for what kind of data you want to capture for a site, that depends heavily on what you're trying to accomplish. We do have a few recommendations around useful data sources for a variety of purposes, and for that I'd recommend checking out the Data Layer Wizard (
https://community.tealiumiq.com/posts/783558). This tool has a number of Data Source bundles that contain data sources commonly used for certain applications. You may find that one of these bundles addresses your needs, at least partially.
I hope you find this helpful.
Thanks,
Seth