"Migrating" to the US is not as simple as it sounds. The US is indeed a land of opportunity for many but they also have one of the strictest and conservative visa requirements.

H1-B is a working visa that allows skilled workers to work in the US. However, this highly competitive visa is sort after by many candidates each year (although 2009 was not filled). This visa needs to be applied by your employer, meaning you can only get this visa if you already have a job, and of course your employer is willing to file it for you.

Green card actually requires you to be staying in the US for a certain amount of time and is not the typical route one will take to work in the US.

Of course there is lottery as well, but that's a whole new story.