We've seen Rey use the Force in TFA and TLJ without any proper training, but at that point she was already aware of the Force and its existence.
What about someone who didn't even know such a power existed? Would they need to understand and feel the force before using it?
(Note: this answer is from a Legends standpoint, though some of the examples hold true in canon and I believe the conclusion does as well.)
No, you don't need either knowledge or understanding. It's possible to wield the Force not knowing what you're doing, although (as you might expect) results will not generally be as refined as someone who's familiar with the technique.
I'll start with the second part of the question: understanding. Understanding the Force is a tricky thing to be sure: there are plenty of Jedi who'd be the first to say that they don't understand the Force, and neither do the rest of the Jedi. But for the purpose of the question, I'll take this to mean an understanding of the Force in roughly the way the Jedi describe it, as an all-encompassing vital force. Can it be used without that understanding?
The answer to that is a straightforward "yes". Groups of "Force adepts" - usually shamans and the like - are found in various cultures. These groups know they have powers, and they can refine and shape the form those powers take, but they generally have little or no understanding of the greater nature of the Force. They cannot "see" the connections between people that make up the Force in the same way that Jedi learn to.
The best-known and explored of these groups are the Witches of Dathomir. The Witches understood that there was power in the world, which they manipulated and directed through spells and rituals, but they did not have any knowledge of the Force's transcendent nature. (As I recall, they vaguely believe that they are the source of the power, rather than using power from an outside source.)
However, while they might be in the dark about some aspects of the Force, and might not understand the full implications of the power they used, the Witches and other Force adepts clearly knew they were doing something. That brings us to the second part of the question: knowledge. Can someone use the Force without knowing that it exists at all?
Again, the answer is clearly "yes". There are numerous instances of people, even children, using the Force totally on instinct, with no conscious knowledge or effort. Anakin himself subconsciously uses a crude form of danger sense, giving him the precognitive reflexes needed to survive in the high-stakes podracing circuits. Before training as a Jedi, Streen had such a powerful gift for Force mindreading that he actually couldn't turn it off and became a recluse. People like Corran Horn had intuitive leaps that they believed were normal but were actually the product of Force sensitivity. There's even a charming image I sadly cannot source of a young twi'lek telling a stormtrooper to talk to the Force hand. None of these people had any kind of knowledge of the Force, formal or otherwise; they just acted on instinct.
WEG RPG material makes a distinction between Force users and people who are simply sensitive to the Force.
Force users know what the Force is and often have been trained in its ways, albeit usually very poorly during the Civil War period depicted in the original trilogy (Episodes IV, V and VI). They have knowledge of the Force, of some of the ways it can manifest itself, and also how to use it to some effect.
People who are sensitive to the Force may never have heard of the Force, or simply dismiss it as a legend or folks tale, and cannot use it in any conscious ways. But, sometimes, they'll have a stroke of luck or an intuition that helps them in some way and they'll always be able to tell right from wrong intuitively.
Thus, someone needs to understand the Force and feel it before being able to really use it in a conscious and directed way.
In Legends(1), vornskrs are animals that use the Force as a hunting sense. Being non-sapient, they can't strictly be said to understand the Force or to be aware of its existence. They simply use it.
Similarly, ysalamiri — also non-sapient — evolved a defense against the vornskrs' Force-sense. It's not clear whether the ysalamiri have any awareness or control regarding their own ability.
(1) Vornskrs first appeared in Timothy Zahn's Thrawn trilogy.
