Find a company willing to sponsor you with a h1b visa. http://www.hooyou.com/h-1b/

There are many ways for you to find long term or permanent employment in the US, but here are some of the main ways that I am familiar with.

1 Study in the US. Attending school in the US provides you with many networking and on campus recruiting opportunities. If you're skilled enough, or a company is desperate, a company will sponsor you with a h1b visa, and in time, that h1b visa will transition into a green card. Keep in mind though, that unless you are talented enough to warrant competitive pay with Americans, you will not receive equal benefits, treatment, or pay that Americans receive for doing the same work or job. Most h1b visas, until they get their green card or citizenship, are paid less than their American counterparts, and it's for this sole reason that they are hired.

2 Join a Korean company. This advice is not really for you persay, but for the OP(Who was very very vague). I've seen numerous Koreans come to the US through a Korean company(located in the US) who sponsored their visa. Usually, during your first year, or first couple years here in the US, the Korean company will keep you as an intern or part time worker and pay you less competitive wages(while also providing you with no benefits), but if you prove your worth with the company, most companies will sponsor your green card/citizenship. After that, it's up to you to negotiate better conditions or go somewhere else.

3 Transfer from a foreign company's Korean branch to their US branch, or transfer from a Korean company's Korean branch to their US branch.

I've seen people who have worked for US companies in their Korean branch like Oracle and Google, who have transferred to the US and were granted green cards after having worked a number of years. I've also seen people do this with European companies.

4 Hire a headhunter. There are headhunters out there, whose sole job is to match foreign workers who want to work in the US, with companies who need their skills and are willing to sponsor them with a visa. I personally don't know any, but some searching around should lead you to them.