I'll admit, I'm a bit rusty myself. I guessed on a few, as noted by the "?" at the end of some of them...
2. Kryptonite and "magic"
3. The meteorites from Kyrpton were radiated by the Sun or the Earth's atmosphere when crashing on to Earth? Some shit like that
4. As a baby, he was put into a ship and sent off by his parents as Krypton was dying
6. The yellow sun of our solar system
7. Jor-El and Lara (with the exception of his adopted mom and WW, every relevant female in Clark's life has an 'L' name, to my knowledge)
9. Ursa and Non
10. The Phantom Zone
14. Jonathan and Martha
15. Kal-El's ship crashed near their farm
16. Smallville, KS
17. Lana Lang?
18. The Fortress of Solitude
19. The S symbol was found on the clothes he was wrapped in as a baby?
20. Martha Kent?