By now you have probably noticed the cave crystals in the caves that spawn everywhere. To start with witchery you will need an orange one. For now, it's not a problem if you can't read the name, but if it annoys you, you can try to progress in ChromatiCraft.
You can use this orange crystal shard to craft a Witches Oven, which is one of the main items you will want to make for early game Witchery.
The other item you will need is a Witches Cauldron. For this you will need to craft anointing paste, and right click it on a Cauldron.

When you have both the Witches Cauldron and the Witches Oven you can craft Mutandis. For this you will need Mandrake Root.
It is highly recommended to harvest Mandrakes at night, otherwise some less than optimal things might happen. Nothing destructive however.

Mutandis is required for Botania, but it can also be used to mutate plants. You will need to mutate plants into at least a Rowan Sapling for now.
With this you can get a Whiff Of Magic, with which you can start Aura Cascade.

The next step will require some Thaumcraft progression. To get the Altar, you will need some Nitor and some Arcane Stone. Both of these should be quite easy if you started Thaumcraft.
The other components are simply gotten either by making the correct sapling with Mutandis or by smelting some things in a Witches Oven, both of which should not pose any problems.
The Altar is your main source of power in Witchery.

The first thing that you will need to have an Altar for is a Distillery. This drains minor amounts of power from the Altar, but it allows for some more complex items to be made.
The most important of these items is Diamond Vapor, seeing as it is needed for the Blood Altar, one of the major progression gates for the midgame.