I have seen this question asked so many times this last week. Usually followed with "oh, and I don't have much money, so they need to be cheap."
Look, if you don't have much money? Don't waste it on herbs and oils you may never even need, especially if you don't even know what they're used for. If you don't have an actual need and money is tight, put that money to better use! Witchcraft isn't just owning things just for the sake of having them, after all. Not that I can blame beginners for this, when so many books make it look like you need a whole shopping list and a half of junk.
Besides, sometimes the best place to start is your own kitchen cabinet. Most people have at least a few bottles of herbs and spices in their kitchen, as well as things like salt, pepper, honey, sugar, and other such things. Check out what you own, how it can be used. It's a good place to start, and if you want to branch out from there, you'll have a better idea of what sorts of things you don't have, or where you'd like to go with your herbalism practice.
Another reason I have an issue with this question is, well, need is just so different from person to person. The herbs I find absolutely essential to my practice probably aren't going to mean a lot to someone else. I try to keep a stock of mugwort, barley, sumac, crown vetch, wild carrot, juniper, pokeweed, foxglove, and mint. And, you know, some of those are toxic, I'm not about to recommend those to anyone just starting out! The others, hard to find, things I harvest locally. Things I use in my spirit work, or for my deities. They can be used for other things, sure, mint is an easy one to find, often cheap - but dozens of other things could be used for such purposes. Why specifically recommend mint, when something else could better fit a need, if a need actually existed? Beyond that, someone looking at that list might wonder where the sage is, since everyone else seems to use it, and yeah it's a "must have" for a lot of people... but I use it very rarely, once a year at most.
So, yeah, if you're going to ask this sort of question, first ask yourself what you actually need, what would actually benefit your practice in general terms. Are you looking to do a lot of protective work? Looking to bring in prosperity? Fertility work? Pick a subject, narrow it down a bit, and see what you have that could work, or what would be easy for you to get, what makes sense for you, because it's not going to be the same for everyone. Put in a little work and decide what you want, what you'll use, and go with it.