It has been established that no one knows why there are some people with magic (Wizards/Witches) and some without (Muggles). But because there is a clear difference, it is logical to assume that there had to be a "first" Wizard/Witch, just as there was a first muggle and a first human. (Which came first, Muggles or Wizards?) However, aside from some basic, uncontrollable magic that exhibits itself in magical children around the age of 7, magic is nearly impossible without a wand. It is unclear whether wizards can Apparate without a wand; Parseltongue is more in the language category so you don't need a wand for that; and Harry accidentally made the glass in the snake cage disappear at the zoo, transported himself to the roof of a building, and made his hair grow back overnight without a wand (that falls under the "uncontrollable magic in young wizards" category), but Apparation is difficult and requires intense training, Parseltongue is very rare, and, as established, magic in young wizards cannot be controlled because they lack the proper education. So, the first wizard may well have known he/she was a wizard/witch, but how did they harness the magic and control it? Modern wizards do so with a wand, but here's another little twist: Wands are made of some sort of wood with a magical something in the middle (dragon heartstring, phoenix tail feather, or unicorn hair being most common). Given that the wood is seamlessly connected to itself, I would guess that magic (i.e. another wand) is necessary to making wands. But if the first wizard didn't have a wand and needed one...? So, how do wizards today have wands and know how to use magic correctly? Given, Tom Riddle did learn to control his magic, use it knowingly and with intent, and to do so without a wand (and however did that happen?), but that was rare. Maybe wizards existed for centuries before someone similar (but potentially less sadistic) to Riddle learned to harness it? If that’s the case, could the sudden influx in controlled magic have spiked the witch trials prevalent in the Middle Ages? How would it have contributed to Muggle-Wizard relations and tensions?
Alright, I’m done rambling. Go ahead and add more questions or speculate on mine, if you want.
“So, the first wizard may well have known he/she was a wizard/witch, but how did they harness the magic and control it?”
Well, as you pointed out, Tom Riddle did know how to control his magic, but you said “That was rare.” Not really. Lily Evans and Snape did, too, and while they were a fairly competent witches/wizards, there were others just as good.
Also, here’s something else to think about. We know four people’s stories before they go into hogwarts. One is neville’s, who is terrible at magic. He was almost not admitted into hogwarts, so we cannot expect him to be super good with his powers at first.
Then there is hermione granger, who as we see, had already mastered some easy spells by the time she boarded the hogwart’s express. The books never go into how well she was doing before then, though, but we can imagine that she already had pretty good control over her magic.
Then we have harry Potter, who was locked up in his cabinet whenever he did magic, so obviously he didn’t put a lot of effort into making himself better.
Finally there is ron weasley. He grew up around magic, but his family made no effort to harness his powers ahead of time, as is shown by his attempt to turn scabbers yellow in the first book. I think ron’s family didn’t encourage practicing magic without a wand, and Ron thought that wands were necessary, so he never tried.
So I actually think that anyone, with a bit of time(we can’t really expect all eleven year olds to have a much intelligence as Tom Riddle, can we?” can control their magic.
“Given that the wood is seamlessly connected to itself, I would guess that magic (i.e. another wand) is necessary to making wands”
Probably the wands were first just magical objects and that they evolved into wands, so the better the wand you had the better you could make one.
And as for the witch trials, I think it’s sort of the same thing as the wands. The better you are at magic, the more likely you are to be burned, but by that time you can do a spell to help you.
That was long. I guess when it's about Harry Potter I can argue forever.