1. "Totally." Like when you're agreeing with someone, you might say, "Oh yeah, TOTALLY." Millennials don't really use it like that. It's more of a Generation X thing.
2. "Sweet." Some people in their late 20's and 30's think EVERYTHING is sweet. Like, "We have next Friday off? SWEET!" So try not to use it too much.
3. "Cool." Younger people DO use it in specific situations. Like they might say a new gadget is "cool." But they don't use it on its own as much. Like if someone says they'll email you, and you say, "Okay, cool." Apparently that's NOT cool anymore.
4. "Bummer." It sounds dated, like something you'd say in the 90's. "Dude, bummer."
5. "Awesome." Again, you CAN still use it. Like if your recent trip to Hawaii was "awesome," that's okay. But it's starting to feel dated, kind of like the word "rad." So try not to use it all the time.