Smaller stones and rocks are not really a problem for most plants, although they can make it difficult to dig into the soil, especially with a tiller. You can remove larger stones if you wish. If you have a solid layer of rock under a shallow layer of soil, consider using raised beds for planting. These can have formal sides or can simply be raised areas with natural edges.
Adding organic matter is the best way to improve garden soil. You can use a good quality compost, well rotted stable or barnyard manure/bedding, old rotted down autumn leaves, herbicide free grass clippings, and so on. If you have access to autumn leaves you could shred them and dig them into your planting area now, by spring they will be partly decomposed and ready to plant.
Loosen the soil and work the organic matter into it, then use an organic mulch year round. The mulch will break down slowly over time and continue to enrich the soil.
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