If you have already worked through the tutorials for Autodesk InfraWorks 360, you will already be familiar with these steps. To get started, you sign in and then open the sample model.
It should be one of the first on the list. The name should have the date of the current release, for example Roads Tutorial (September 2015).
Use the default location unless you have a particular reason not to.
The download will take a few minutes at least, depending on your internet connection and various other factors. The model will then open automatically.