To get started with Botania, you need to have done enough Witchery to get Mutandis.
With this you can get a Petal Apothecary, which allows you the get a Pure Daisy.
The Pure Daisy can be used to get Livingrock and Livingwood. Those are required for mana storage and transport.
The final thing you will want to get are a few dayblooms, and then you can get to producing mana.

Your goal for now is to acquire some Manasteel and Mana Diamonds.