You had a crappy family, and a shitty past, so, the day you turned 16, you took the few possessions you wanted and joined the military.
Once you're in they start you as a normal recruit, but they quickly notice your skills, and you're put on the radar for special forces.
Price had been looking for new recruits for a mission against Makarov, they'd recently lost one of their best soldiers, and that soldier had an important job... to stealthily break in while TF141 created a distraction, and retrieve the nuclear launch codes secretly. That's when he came across you, you had a perfect score in all your recruitment assessments and continued to get high praises from your ranking officer. Price wanted you, but they didn't have the time to incorporate you into the team and do normal training, they needed to test you, and they needed to do it immediately.
You're randomly transferred to Iraq, not sure why, but you ignore it, knowing to listen to your ranking officer. What you didn't know is TF141 planned to ambush you, and if you could last five minutes against them, you'd be recruited. But, you had to do it while also being shot at in an active battle.
You take your shots with precision, before joining the army you were a hunter, you took out all your kills with a single shot through the eye, which perfected your aim completely.
About to take another shot you smell something, mint... someone stepped on a mint plant, you take your shot and wait for the attack, the fact they're staying hidden suggests they want to do this silently, so they won't be using a gun. So you continue to take out enemy soldiers while waiting for the enemies to make their move.