How do you unlock weapons in team fortress 2?

In TF2 you earn items by playing for a randomly generated amount of time, there is a tiny chance of earning a hat every time the timer hits "0" but you usually get an item. There is no specific task you must complete to earn the flare gun. Here are the several ways you can obtain it.

1. Random drop (every once in a while, you get a random item for free, there's always a chance you'll get the Flare Gun)
2. Buy it from the Mann co. shop. (not recommended)
3. Earn 10 pyro achievements.
4. Get it through trading.