These are my top tips for new players starting Crusader Kings 2.
1. Read the Manual or watch some videos on how to play. Quill18 has some good ones.
2. Be the big fish in a small pond. Start in a quiet spot and build up. Ireland is considered a good place for beginners.
3. Use the search tools in the bottom right to help you find a place or person.
4. From your portrait, access the realm tree to see how each vassal feels about you.
5. Make use of the wiki. It was from here I found out that your heir may plot to assassinate you below an approval of 25.
6. Go on crusades. Sending your armies off on Crusades will yield massive rewards.
7. Don't start a holy war when surrounded by the opposing religions nations.
8. Gift gold to your vassals. Their approval will determine how well your realm functions
9. Check the ambitions for those at court. Arranging marriages is an easy way to get approval up.
10. Plan your heirs marriage well. Sending them off to another court could lead to a loss of your territory.
Bonus. Tutorial from Quill18 for base game at: https://www.youtube.com/watch?v=sMtkcXS3QJM&t=1945s