How do I choose where to start?
Energy will follow need and interest. So even though, from a practical standpoint, you could start anywhere and build toward a balanced program, your most pressing needs will be a reasonable starting point.
Many congregations, for example, are concerned about declining membership. Membership Development, therefore, will respond to felt needs. At other congregations, where membership seems too "gray" for long-term vitality, a focus on Young Adults Ministry might make more sense.
Two points to consider:
First, in order to assess your efforts, you will need measures of what is working or not working. Therefore, some attention to Metrics will be necessary wherever you start.
Second, virtually any best practices that you choose to implement will require a capable web site to be effective. While you work on, say, Membership Development or Leadership Development, it would be good to have a parallel effort to implement a portion of the Communications Strategy section, namely, a web site that uses best practices in design and features.
Remember, finally, that your starting point is exactly that, a starting point, not a full project.