There's about a dozen different plants to pick. Off the top of my head: daisy (chamomile), poppy, marigold, sage, nettle, comfrey, st john's wort (the yellow one), mint, belladonna, wormwood, eyebright and a few different mushrooms. You should be able to pick all of these from the very start. You don't unlock new varieties, they're available from the get go. Take some time to learn what each one looks like. Belladonna and wormwood fetch the better prices, but you won't make a lot of money. You can however unlock different abilities to do with herbalism like fetching better prices and gaining strength. As you level up in the skill, you'll begin to pick up more plants in one go and this can be very handy if you're looking to gather a lot at once.
As for brewing, you won't be able to do this until Henry learns to read, as you'll need to follow recipes, so save enough to teach him the skill. The scribe in Uzhitz will teach him. Buy/find/steal whatever skill books you can. Take time to learn sword and bow skills, lockpicking, pickpocketing and horse riding. Opening chests (especially the 'warhorse 5') can be very lucrative. Find as many maps as you can (warhorse 5 you get at the mill, there are 25 regular treasure maps aside) and sell what you don't need- this is the best way to make money.
You may find it hard at first (especially horse riding), but you'll eventually get the hang of it. This game is an absolute gem and one of my favourites. I hope you enjoy it!
