I personally feel that a good way to understand and tap into the magical energies flowing through herbs is to first study and understand the herbs themselves.
For example, every herb has it's own flavor, scent, texture, and growing habits. These qualities can actually help you determine the magical properties that a certain herb might have.
Let's talk about garlic as an example. Garlic is a very versatile herb. I've found that it grows well in both sun and shade. It is a very spicy and hot herb, therefore, it is associated with both the element of fire and the plant Mars.
In magic garlic is used as protection against the evil eye, protection against evil malevolent spirits and other negative forces and energies. It can also be used in sells for healing and banishing.
Ginger is spicy with hints of sweetness. It can be used in spells for protection, love, passion, etc.
Chamomile has a sweet and soothing flavor/scent with small hints of citrus, and is often used in spells for sleep or to promote relaxation.
If you have the time and wish to do so, studying your herbs can tell you their magical uses by their flavor, texture, scent, and growing habits. Working with them in such a way is a wonderful method of getting to better know and understand them and what they can be used for.
I can also mail you several good books that list magical associations of herbs, if you would like.