The road to a healthy yard begins with at least a year of careful observation and note taking. Where does the sun shine and where is it shady in the various seasons of the year. Are there micro climates? What is presently growing there? What is the history? Any chemicals used on the property -- herbicides, pesticides, insecticides (especially chlordane which was once popular against termites and contaminates a lot of urban properties)? What about lead in the soil from lead based paint and the days of yore when lead was an ingredient in gasoline?
Resist the temptation to start doing a bunch of unconnected stuff. If there are serious issues, like erosion in process, go ahead and take some action like putting down mulch or creating some check dams or planting something to hold onto the soil, but generally, action without observation often turns out to be counterproductive. It's not possible to grow a healthy yard in a "fit of passion" or "one quick season of work." As you observe, evaluate your observations and think about your goals, needs, and wants with this yard. Then you make a plan and begin to implement it -- Observe, Study, Evaluate, Design, Implement (OSEDI is the acronym).