The best way to learn potions, or what to use is to (provided you have completed it) to possibly invest into alchemy enough that eating will teach you all four ingredients then *spoiler* to reset your perks in that tree to move somewhere else. after you have all your ingredients figured out, you can work from there. If you are looking for something in a very specific scope or range, you can always use a guide of some sort as hunting recipes may be a hassle. If you opt the "easy" way out and look for a guide, you can use the resources on this site (we have two guides dedicated to Alchemy) or the vast resources of Google's search engine.
As Jake said, be careful of using a third ingredient, sometimes it can barely affect the potion, or sometimes it can drastically change your smithing potion into a poison by accident.
Overall, if you completely wing it, just harvest a bunch of ingredients (and i mean a lot) and just sit at an alchemy table and have a field day. Sure you'll probably waste some money or ingredients but you'll learn some new potions you didn't know before.