Using View's Relationships is quite simply a matter of knowing how "one piece of content relates to another". If you can wrap your head around this simple concept, then all your complex Drupal views will be realized quickly.
The most common use of Views relationships will center around content types which utilize some form of "reference" field type. This will likely be either User references or Node references - but won't be limited to these field types for use in Views relationships.
Understanding how the data is stored within the Drupal database is also a big help to achieving your goals.
If you start with the data and relationships in mind first, then arriving at your ultimate goal will become much easier.
