Answer: Early spring and fall are the best times to plant for the best results with the least work, mainly because summer heat is so stressful on plants and the dry spells mean extra special care must be taken with watering. Early spring usually means the widest selection of plants will be available, too. However, most container-grown plants can be planted any time the ground can be dug -- if you are attentive to their needs.
Soil preparation is very important to giving plants a good foundation and helping them to be more self sufficient in the long run. You will need to run some basic soil tests and work with the soil as indicated by the results. This can be done now as it should be done several weeks in advance of planting so that the soil can have time to settle.
Since you are new to gardening and still in the planning stage I would strongly suggest you look at a book or two about general gardening and landscape planning. Your plant selection will be best determined by analyzing the microclimate and growing conditions in your yard and then selecting plants based on what will thrive in the conditions you have to offer -- for example, shade, part shade, part sun, full sun, windy or sheltered, naturally dry soil or damp, naturally rich or poor soil, etc.
There are several books in the Dummies series that you might find helpful. "Gardening for Dummies" and "Landscaping for Dummies" are excellent practical guides to get you started and include information about soil preparation, plant selection, as well as the actual planting and maintenance.
Once you familiarize yourself with the general principles, then your local nursries and county extension should also be able to help you with specific plant suggestions based on the conditions in your own yard.
Good luck with your new plantings!
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