Growing a Witch's Garden
When you are planning your witch's garden, ask the spirits of the place for help in siting it and keep in mind the interaction between human and non-human a garden entails. Make it a comfortable place to be, bring your magical practice into it (and allow it to enter your magical practice), start small and know your limits, and make appropriate sacrifices. You should understand the sanctity of garden work, keep the motions of the heavens in mind, and take a broad view of your garden's productivity. That's a bunch of shoulds, but the final should is that a garden should be a delight.

Siting Your Garden
Try to open yourself to the spirits of the place before you site your garden. You want a good site physically for the plants, but also spiritually. You can do this simply by wandering about your potential garden area at different times of the day and opening yourself up to what is there. You should get a feel for which areas will benefit a garden. Full sun is preferred for many gardens, but it's possible to make a wonderfully magical garden in shade, since many witching plants are not only shade tolerant but actually prefer shade, like belladonna. In fact, I wonder if part of the attraction to some of these herbs for witches in the past was that they could grow in areas that might not be so easily visible to neighbors or passers-by. Herbs growing around the doorway are quite a bit different than herbs growing in the edge of the woods or hedge bordering the witch's property.