Business Insider posted a list of things that make you seem rude. If you do any of this, you should have learned better when you were younger. These are things a child would do, but apparently adults do too.
1. Not saying "thank you." And writing thank you NOTES isn't a huge thing anymore, but it should be.
2. Interrupting people. Most people don't do it on purpose, and might not even realize they did it. But the person you interrupt definitely does.
3. Asking questions that are too personal. Especially at work, or with someone you're not very close with. It puts them on the spot, so they feel rude if they don't answer.
4. Not having good table manners, or being a jerk at restaurants. So everything from chewing with your mouth open, to yelling at a waiter. Another one is ordering the most expensive thing on the menu when someone else is paying.
5. Checking your phone when you're talking to someone. You seem like you're not really listening, and don't care about what they're saying. A lot of people even think it's rude to leave your phone on the table in front of you when you're in a meeting.