Determining general rules for immigration is a challenge since there are so many different rules depending on your situation. In most cases you will need to get a visa to be allowed to immigrate. There are actually more than sixty different types of visas available that allow residence in the US so your situation will largely determine which approach you can take. That being said there are a few categories of visa that most people will fall under.
For most immigrants that ultimate goal is to get a green card, this is an essential step on the road to citizenship. The green card is also what grants you permanent residency status. There are basically four ways that you can get one, the first is because you already have family living in the US. The next option is because you are offered a job; in this case your potential employer will have to be able to show that there is no American who can do the job. You can also get a green card as a refugee or you can win the green card lottery.
While getting a green card has to be the ultimate goal if you are planning to immigrate to the US you will likely have to start off on one of the other visa programs. The easiest to qualify for will be through your job. There are number of different visas that will allow people to work in the US if they have skills that are required. These are normally only temporary visas but if you maintain your job for a long enough time you will be able to apply for a green card.
In general if you already have family living in the United States you will be able to get a visa this way as well. Keep in mind that family means close family not a distant cousin. Generally it is intended for spouses and children of people already in the US. This is another program that can be used to qualify for a green card. In most cases you will need to be able to demonstrate that you have been able to maintain a job for a minimum period of time in order to get the green card.
One of the ways that you may be able to get into the US is through the green card lottery. This is done by country with a certain number of spots being available for people from each country based on a formula that the Department of Immigration uses. In order to be considered you will have to meet a certain minimum requirement, usually you need to be able to demonstrate that you are likely to be able to get a job. All of the people who meet the qualifications are entered into the lottery and the winners can move to the United States.