If you are unsure which part of the plant to use or how to preserve it I'd suggest you do more research on herbs until you have a handle on that. In addition, are you trying to grow medicinal herbs or magical herbs? That can make a difference in how you go about it. Here are some books that I'd suggest:
"The Master Book of Herbalism" by Pau Beyerl
" Culpeper 's Complete Herbal: Updated With 117 Modern Herbs" by Nicholas Culpeper and Raven Starhawk Cunningham
"The Practice of Traditional Western Herbalism : Basic Doctrine, Energetics, and Classification" by Matthew Wood
Also, you might check around in your area to see if that are any herbalist societies. Becoming a member is a great way to learn from those who have been practicing herbalism for a long, long time.