How did I do for the first time using a flash?

untouched photo, right off of the camera.

I'm no expert, I'm learning OCF myself, but there are a couple things that stick out to me.

First the eyes look nice exposure wise, but I would like to see more catch light in his eyes.

Second, I find the shadow under his chin a little distracting. It makes his chin look longer than it really is.

I think both could be fixed with different light placement. As to what that placement would be, I have no idea. That is what I'm having the most trouble with. I have twin 4 year olds, so their attention spans aren't that great. I get maybe three or four shots of each and they are done. In those 3-4 shots I try to get an idea of best lighting position, but I'm struggling with it.

Direct, straight on fired, on camera flash produces flat lighting and harsh shadows. That's what you got here. Get your light source off of the camera for better flash results. Use some sort of modifier (if only a small shoot thru umbrella) for even better results. Not being harsh, but direct on camera flash just isn't flattering to people/faces. You don't want a hot shoe mounted flash as the main light source on faces.

It's really nice picture (in terms of straight off the camera)
But what the other two people have said, the flash makes the picture really "not good" per se.

You could try to use a diffuser, like one of those cheap ones you stick onto your on camera flash (i'm not sure if that would work, but you could try!)

The harsh shadows kill a perfect pose. Try a diffuser or getting it off camera as mentioned above

Nice smile! Handsome young man! As others have mentioned, from a portraiture point of view, the flat lighting from direct flash is not the most appealing. I also think the composition could be improved. It looks like a nice setting. I would have moved him away from the tree.

Tim