Hi @transendence,
Yes, you are right. You can use herbs, crystals, and everything else to make potions, elixirs, oils and stuff to assist you in working your magic.
As to what herbs/crystals to use for what, is kinda difficult to say, since everyone of us has a different connection to the items they use. There is also no right or wrong way to create your potion, as it depends on your intentions, your level of experience/knowledge of used ingredients, and more. It’s a very personal thing.
For example: I recently created my “Serenity Oil” to help me stay grounded, calm and focused using Valerian flower, Lavender, Rose Petals and St. John’s Wort. Since it’s going to be an oil I put on my body, I used Sweet Almond Oil as the carrier oil.
Some witches include brick dust or graveyard dirt for protection, others just use sage, clove, cedar or rosemary. I hope you get my point.
Research is mandatory, not optional, since you can harm yourself and others by using poisonous plants or parts.
Start by researching the properties of the things around you. To get you started, here is a correspondence chart for herbs/plants:
and for flowers:
But there’s a plethora of knowledge on the internet. And it would be way too much information to post every herb, stone and their correspondences and properties here. Planetary, zodiac, color, etc…
Read through it, connect with the item at hand and see what it tells you to be used for.
Hope this was in any way helpful for you. Feel free to follow up or reach out if you have any questions