Botania is a mod centered around the use of flowers to do certain things. The first thing you should do to get started with Botania is look around you for flowers called Mystical Flowers, they all have a colour (e.g Cyan Mystical Flower) They are easy to find once you recognise them and WAILA can help with that
Once you have a starting base, you must craft the Lexica Botanica. This book is the mod's guide and can be made with a book and a sapling (shapeless recipe) This will answer all your questions about the mod, in which you can:
Create flowers with petals from the flowers you found that can:
Use mana to do many things
You can make large mana networks with the help of Mana Pools and Mana Spreaders
You can also use mana to make various other items.