There are two main types of visas - "immigrant" visas, or green cards, which allow you to live in the U.S. permanently; and "non-immigrant" visas such as student and tourist visas, which don't. How can you get an immigrant visa, or green card? If you don't fall into any of the following categories, there is no general line to join for a green card!
Family reunification: some categories have per-year and per-country quotas and some don't.
There are no quotas for a spouse, single child under 21, or parent of a U.S. citizen, although it can take months or even years to get through the paperwork and processing. There ARE per-year and per-country quotas for other categories of family members. It can take 20+ years for Mexican or Filipino siblings or adult children because there are so many applicants from those countries. Some family members, such as cousins, grandparents or grandchildren, and same-sex spouses, don't qualify at all.
Employment - there are no waits in some skilled employment categories, but others may take 10+ years. Again, there are per-country limits, so Indian and Chinese employees often have the longest waits, even though their skills are needed.
Refugee or asylees - can apply for a green card 1 year after their entry or grant of asylum status, and there's no quota. They must prove persecution on account of race, religion, nationality, and/or membership in a particular social group or political opinion.
Diversity visa lottery - up to 50,000 visas are granted each year to applicants from countries with low immigration rates, such as Australia, Nepal and African countries. Applicants must meet education, work, and other requirements.
So you can't just line up to enter the U.S. because you want to. Reform proposals center on increasing the numbers and types of visas available. The U.S. places a priority on family reunification, while other countries take a variety of factors into account. A wonderful and thoughtful resource on international migration issues and statistics is the Migration Policy Institute Stay tuned!