Can I use first job points for second job skills? What about the third job?

No. Any skill points you gain in the first job MUST be used before the second job. The same is true of second job points on Third job skills, and so on. It is okay to use second job points for the first job, but this is generally not recommended. Beginner skill points can only be assigned to beginner skills. Some skills, such as monster rider or Legendary Spirit, are automatically maxed when the player receives them.http://strategywiki.org/wiki/MapleStory/FAQs