Mesa,
I would rather not do all your homework but I will give some help.
1) Study the Korean War and pay particular attention to Chinese involvement. Try and focus on why they became involved and what they were willing to sacrifice.
2) Study the history of Korea...emphasize on the geographical significance of it over the ages. I'll throw in "buffer state" to get you in the right direction.
3) Get a subscription to Strafor and study the N. Korean military and their likely strategies should we attack them. How much artillery, tanks and men do they have right next to Seoul. How many millions would die in the Korean hoard raining south? Then look at the social and economic state of N Korea. Is that something we really want to deal with should be conquer them?
4) Oil does have something to do with the Iraq invasion but regional stability and reducing terrorism have a large part. Here, I would like you to google which Arab countries helped us in our Iraq invasion. Aside of Iran and Lebanon, please tell me which Arab countries did not base our troops, weapons or allow overflight rights. Then ask yourself, if we were there to steal all the oil, would these countries be in favor of it? Perhaps Saddam was a destabilizing force there? Perhaps a democracy in an oil rich state with better than average education rates would help stabilize the area thus reduce terrorism.
5) Ask yourself this. Do you think Iran will get a nuclear weapon in the next couple years? If your answer is in the affirmative, what do you think Saddam would have done to counter that threat?
6) Last, read up on the term "state interest."
That is all!