Hi Andre-
The first ground rule is to understand how Revit Building works.... you can download or request a trial of Revit Building from your reseller. Revit Building provides many tutorials on content building, and will get you started in understanding how modeling in Revit works.
Revit Systems does not currently support metric units... but you can learn how to building metric content using Revit Building.
Ground Rule 2- Start with an existing family. For example, if you are creating an Air Handling Unit... don't start from scratch... you'll be making it too hard on yourself. There may be a AHU template, but if not, start with an existing AHU family to ensure you have the correct Family Category and Parameter settings (on the Settings menu)
The next ground rule is to understand connectors in Revit Systems... the main differentiator in Building vs. Systems content. There are connectors for electrical, HVAC, and piping... there are many different properties and options for each.
Another ground rule (more like a tip)... you can also learn a lot by looking at the parametric content in ABS. Although the two formats are not compatible, you can get a sense of what it takes to build parametric content, and how you may define your geometry, parameters, and formulae.
HTH,
Martin Schmid
MEP Consultant
Autodesk Consulting
Martin Schmid
Product Line Manager
Mechanical Detailing and Electrical Design
Architecture, Engineering, and Construction
Autodesk, Inc.