Mulch in the spring, putting a layer about three inches thick over all of the exposed soil in the garden, in between, but not right next to the plants. Leave at least an inch of un-mulched soil around each plant stem. Mulch again in the fall when you're putting the garden to bed. That way the mulch will decompose over the winter and add all that good stuff to the soil. In our area, any time after May 15 for perennials, and after May 30 for annuals.
Perennial gardens take time to mature.
While adding plants to your gardens, allow enough space for your plants to grow and mature. Your garden has a front and a back. Color counts, too. Consider the color both of the flowers and the foliage as you choose your plants. Try grouping plants together at the nursery to see how they look, and ask for advice.