Film (see script quote below) covers that - he tells it to Rey
I'm not Resistance. I'm not a hero.
I'm a Stormtrooper.
Like all of them, I was taken from a
family I'll never know. And raised
to do one thing...
But my first battle, I made a choice.
I wasn't going to kill for them. So
TFA novelization by Foster covers that.
Combat was not at all like the would-be rescuer had envisioned it.
A brief, final flailing by the downed trooper splattered the newcomer’s face mask with blood. Then hand and arm fell, and movement ceased.
There was no assistance to be rendered here, the second trooper realized. Straightening, he surveyed the hell in which he found himself. His weapon hung at his side—unfired. He stumbled off, away from his dead comrade and that exposed, pale, pleading hand.
As madness ebbed and surged around him, he wandered through the village, feeling himself more a participant in a historical drama than in an actual battle. The horrific and all too common red stains on the ground contradicted his denial. This wasn’t like his training at all, he told himself numbly. Unlike in simulations, reality bled.
Now, there's actually a deeper back-story for this, both generally and for Finn.
The Visual Dictionary page for General Hux (p.40) explains that this was a standard thing - he was a firm believer that combat simulations were sufficient and great training for soldiers, following in footsteps of his father who developed this methodology as Commandant of Imperial Academy. I added more details to this answer about Hux
In the prequel book "Before the Awakening", we see the exact timeline.
I put a lot of details and quote about it in this answer. Short exerpt:
- FN-2187 was a cadet in training
- Then, he was called up to deploy first time ever, with Captain Phasma. At that time he was told he was becoming a real stormtrooper, and never saw battle yet
- That deployment was to some Outer Rim system Pressy’s Tumble/Pressylla, and was basically urban pacification (translated: they were ordered to shoot the leaders of striking miners)
- Immediately upon the end of THAT event, Phasma loaded them all up on board SD, and headed to pick up Kylo Ren on the way to Jakku.
Captain Phasma watched FN-2187 on the monitor in her quarters. He’d stopped firing, stopped even moving, and was just standing amid the ever-changing field of moving figures.
She sighed. She’d had such hope for FN-2187. He had shown such remarkable promise. He had shown the capacity to be special.
She picked up the orders on her desk and reviewed them once more. They’d already made the jump to hyperspace, and she knew it would be less than an hour before they reached their rendezvous point to take on their new passenger. Kylo Ren had already transmitted the coordinates for where they would be headed next.
She switched off the monitor. He’d be part of the detail when they reached the landing point on Jakku, she decided. Perhaps when someone was shooting back at him, he would understand what it meant to be a real stormtrooper, what it meant to serve the First Order, body and soul.