How do I get items in Team Fortress 2?

The item drop system gives you a random item after a certain amount of time.

Players are guaranteed to find items at regular intervals of 30 to 70 minutes, with an average interval of 50 minutes.

There is no cap on the amount of items that can be received per week, but instead a cap on the amount of playtime in which drops can occur

Each class also has items that are awarded based on completing achievements with that class in addition to being randomly found.