I usually start by working out what are the key data objects that need
to be represented in the database, and the associations between them
(has_many, belongs_to etc). Possibly User, MeterReading for example,
then build the system up from there. Make sure you write your tests
as you go so that when you refactor things (as you surely will) the
tests allow confidence that the refactoring is ok.
This document (getting started RoR) has many answers to your possible questions. It is best to start with Testing Rails Applications — Ruby on Rails Guides and railscasts.com - a little outdated but has many great examples there.
Start with those, rest will follow