General:
1. A piece of cake: Something easy to do.
2. Break a leg: Good luck!
3. Cost an arm and a leg: Very expensive.
4. Hit the nail on the head: To be exactly right.
5. Let the cat out of the bag: To reveal a secret.
6. Once in a blue moon: Rarely.
7. Spill the beans: To reveal a secret.
8. The ball is in your court: It's your turn to act.
9. To get cold feet: To become afraid or nervous.
10. To pull someone's leg: To joke with someone.
Figurative:
11. See eye to eye: To agree with someone.
12. Give someone the cold shoulder: To ignore someone.
13. Bite the bullet: To face a difficult situation.
14. Put all your eggs in one basket: To risk everything on one thing.
15. Burn the midnight oil: To work late into the night.
16. Get your ducks in a row: To organize things properly.
17. Go the extra mile: To make an extra effort.
18. Keep your chin up: To stay positive in a difficult situation.
19. A blessing in disguise: Something good that initially seemed bad.
20. Cut to the chase: Get to the point.
This list is just a starting point. There are countless other common idioms in English.