Straight: When Kenta's Doomed Hometown is attacked by bombers, he ducks into a warehouse to avoid being killed, finds a Humongous Mecha and ends up falling into its cockpit.
Kenta is able to pilot the Humongous Mecha with ease despite failing his driver's test three times catastrophically.
Kenta ducks into the warehouse, the mecha was stored underground, the planks over it were moved, and he literally falls in.
Downplayed: Kenta is just an ordinary pilot who, due to unexpected circumstances, ends up in the cockpit of a Super Prototype mecha that wasn't assigned to him.
The mecha uses some sort of exotic control mechanism which makes it "bond" with the first person to use it. Alternatively he was a trainee mecha pilot, or at least is familiar with the controls, who happens to be quite good in a pinch.
Alternatively, the mecha's controls were, following one school of design seen in tank controls, designed to invoke this—it cuts down on the time needed to train a raw recruit if the controls are something that will be familiar already & the main thing you need to do is familiarize them with this machine's peculiarities.
Kenta sees the Humongous Mecha, freaks out and refuses to go anywhere near it.
Alternatively: Kenta does fall into the Humongous Mecha, but being a mere highschool kid, can't figure out what anything does and when he tries the mecha either does nothing or it ends up causing system failures, rendering the mecha inoperable.
...But it activates itself and pulls him into it.
Kenta falls into the Humongous Mecha, and has no idea how any of it works... However, the mecha is alive and guides him through the process.
Exploited: They store the prototype in a training area in case anyone tries to hijack it they can at least test them and see if they should be recruited or jailed. Kenta performs like a seasoned veteran on his first try so they recruit him and train him up to true Ace Pilot levels
Defied: By the time Kenta sees the Humongous Mecha, the official pilot is already climbing into the cockpit.
When it's discovered that Kenta is Just a Kid who stumbled across the mecha, he's immediately arrested and hauled in for brutal interrogation, accused of being a traitor, a spy and anything else they can think of. The organization is blasted for letting the machine fall into the hands of some ignorant civilian, and Kenta faces life in prison or execution just for being in the wrong place at the wrong time.
Alternately, Kenta doesn't have any clue how to make the thing work and ends up being horribly killed or killing several other people before he can figure it out.
Kenta manages to survive his first battle and as the pilot is drafted into the army. However, he's not psychologically fit to be a soldier; the stress breaks him, and over time he sinks into insanity and despair, until eventually taking the mecha and trying to kill as many people, allied or otherwise, as possible before he's shot down.
Kenta's Mission Control is online to talk him through the start up procedure, what the controls do, etc. as the situation unfolds. All he has to do is follow their instructions.
The Captain personally makes his way to the mecha, pries Kenta out of the cockpit, and takes him for therapy and some proper military training. When he returns to action, Kenta is a more skilled pilot and has largely gotten over his psychological issues.
Played For Laughs: Kenta asks for a Humongous Mecha for his birthday, he ends up actually getting it.
Played For Drama: Everyone around him is dead, dying or in mortal peril; their possible survival hangs solely on whether or not somebody can get the Humongous Mecha up and running to defend them. But Kenta's the only one left living where the mecha is stored, and he only just found out about it — he doesn't have any idea how it works! But he needs to figure it out, and FAST!