I'm trying to understand it a bit more but i got a few questions. I haven't bothered with alchemy all this time until now.

I got a few ingredients but nothing really interesting except for Vampire dust which gives silence. However, when i use my Apprentice Mortar & Pestle, i can't do anything with the Vampire dust. How come?
I'm also trying to find ingredients that do health damage and paralyze, a friend told me how cool it is to make a poison of that and apply it to an arrow. I checked the Wikia and it said that the Milk thistle seed should give paralyze but it only gives light. I don't fully understand alchemy yet, Ectoplasm for example...i got 7 of those, but i can't create a potion/poison out of it. When i select 'add ingredient' and i press 'Y' for All, i can select any other ingredient, but i can't create it. I don't get it. And can i buy poisons somewhere?

Do you have another ingredient with silence as the first effect? If not, then that's why you can't make silence potions out of one ingredient by itself. Your skill is too low at this point for those perks. If you want access to more effects for each ingredient, you have to increase your alchemy skill.
In order to make a potion or poison, you need two ingredients that show the same effects to make one with that effect. A poison is made when a harmful effect is produced. A potion is made when a beneficial effect (to your character) is made.

As for buying poisons, it's a spoiler, but try