First read the documentation. That is usually a good place to start.
Or, more generally 'getting started with topologies' seeing as Geometric Networks are a kind of topology rule set. You should have access to this training if your Org has an active ESRI license.
Without knowing the context, working for a Water Utility we are starting to talk about long term plans to switch off of the Geometric Network and switch to the Utility Network which is ESRI's new solution to network connectivity and (I believe) is intended to replace the geometric network.
more links from a simple google search:
Tutorial 5-2: Creating geometric networks
Exercise 5: Building a Geometric Network