as Duchess said, the ingredients are filtered, but once you add one, when you click on the slot to add a second, it will only show those that can be added.
to make a health potion, simply choose a base ingredient that has
restore health as an effect (the effects are shown when you mouse over the ingredients) and then add another ingredient that also has restore health as an effect.
when making health potions, be careful not to add ingredients that also have matching negative effects.
but don't worry about making health potions, you'll find more than you need. just gather and buy ingredients then mix potions and sell them to make big money quickly.
when ever you see any food laying around unobserved, take it and then make potions from it as soon as possible as that gets rid of the stolen status and you can then sell the potions to legit merchants
(this was the only regular theft I found profitable as it the items sold for a better money than most things)
I had to remind myself how it worked as I had recently had another look at Two Worlds and that has a different (and better IMO) alchemy process