The original Nova - at least the hero one in the comics - was an earth kid. A high school student named Richard Rider. In this story, a kid from Queens stumbled upon a dying Rhomann Dey (the Nova Supreme of the time, if memory serves) who was on earth on a quest when he got ambushed. Dey gave his uniform (and the powers that go with it) to Rider and Richard became A Man Called Nova.
Next up, 15-year-old Sam Alexander picked up his dad's helmet (with a little help from Gamora and Rocket). Seems dad was a retired Nova Centurion wallowing in booze in Arizona (town named Carefree - of course!). In this case, Gamora and Rocket did the training and Nova was ready for prime time.
But, in the current context of the MCU, I don't think Marvel has any retired Nova Centurions lying around boozing it up. I don't think the Nova Corps has even really taken notice of our fair green and blue ball.
Now, Define Him
So, I'd go with Richie. Peter Parkerish high schooler who may be a football player - or baseball... soccer? Give him some cred outside the U.S. of A. Yeah... make him a decent soccer player. Can't get any playing time, though, because there's an all-state stud that plays his position. He has to settle for getting his kicks as the top dog on his club team.
Give him a cute girlfriend - say, Anna Sophia Robb, Hailee Steinfeld, Chloë Grace Moretz, Elle Fanning, Coco Jones, Bonnie Wright, Dakota Fanning, or, hey... some girl off the street - so long as she can act and can be made to look like a real high school kid and not some "plastic". Ya never know - you might want to give her a super power one day... er, besides her looks.
Then, in, say, [The Avengers: Age Of Ultron](movie:293035), a stray Nova centurion is approaching earth looking for the Avengers. They have a report of some powerful heroes appearing on our maverick globe and want to know if they have an interest in forming a partnership with the Nova Corps. He is surprised to see a fleet of ... robots? flying over the city.
Meanwhile, at a high school in Queens, robots are raining down out of the sky; kids are running in terror; the soccer team forgets practice and runs for the gymn; one of the robots takes out a rocket that crashes into the school - same rocket that bears the centurion from Xandarr.
No? Already too much known about Avengers: Age of Ultron? Too close to release? Ok... fine.
How about... [Captain America: Civil War](movie:994409)? Plenty of time for a 5-minute interlude at a Queens high school, right? Sure.
Rocket ship √
Nova Centurion √
Soccer team √
Ok... ok... no time for the girlfriend - for now. Fine.
No robots so no panic. Fine.
Uncle Sam fires on a UFO blithely skimming across the city on its way to whatever exists of Avengers Tower.
Rocket crashes into the gymn √
Soccer team pries open the door to the ship and find the centurion. He's dead. Rider picks up the helmet lying next to him and gets an electric shock.
Finish the movie. Avengers beat each other up... yada... yada...
Post Credits Scene: Richard is asleep and dreams about the man in the rocket ship. He reaches for the helmet... and, there it is! He wakes up holding it!
Now, Implement the Character!
Now, in [Guardians of the Galaxy 2](movie:1081113), he's trying to figure out what to do with this helmet. So, he finally puts it on and it takes him to where the rest of the Nova gear is stashed by the authorities. Quickly collecting that and putting it on, he then follows the helmet's instructions to Xandar. Once there, he bumps into Star Lord who takes him under his wing and graciously "escorts" him to Nova Headquarters. Gotta have a drink, first, though.
Don't worry! This isn't earth!
Wait, you're from earth?
Eventually (fade to new location - don't waste anymore movie time...), Richard finds his way to Nova HQ, but he now has an appreciation for the Guardians. Weave him into the story from there.