These are two questions that seem to be unrelated, so this response will focus on the first. In looking at the issue of immigration, you may want to consider the following approaches. Note that the prompt is asking about immigration in general and not illegal immigration, which is generally the focus of contemporary immigration debate (with the exception of some fringe groups who wish to rein in legal immigration as well).
First, you might want to consider the history of the United States. It is often said that the United States is a "nation of immigrants," which is an entirely defensible position. At the same time, many immigrants in American history have faced discrimination, including Irish and German immigrants in the mid-nineteenth century, Eastern European and Asian immigrants after the Civil War, and Latin American immigrants in the twentieth century. In short, those who argue that immigration poses a threat to American civilization and culture should understand that the same arguments were made by past generations of nativists.
Another factor to consider is the economics of immigration. On this point, the research is clear. A recent study by the Wharton School of Business at the University of Pennsylvania pointed out:
Economic analysis finds little support for the view that inflows of foreign labor have reduced jobs or Americans' wages.
This is a response to the charge that immigrants, through their willingness to take low-paying jobs, drive down overall wages for workers born in the United States. This study finds that this claim is false, mostly because immigrants do not compete with natives for the same jobs and that "the economic effects of immigration are mostly positive for natives and for the overall economy." Of course, these arguments tend to emerge during periods of high unemployment, but historically, immigrants supply a much-needed labor force that is essential to economic growth.
Finally, other factors to consider (remember that the prompt refers to legal immigration) are the origins of anti-immigrant sentiment. Historically, nativists have been of Northern European origin themselves (and have not favored restrictions on immigration from Great Britain, for example). Rather, they oppose immigration from places like Latin America, Africa, the Middle East, and Asia. This position threatens to inject race into immigration policy in a way that is of dubious legality and is certainly immoral and undoubtedly rooted in racism.
Each of these factors is worth addressing in more detail. Some other issues that would be important to consider are the relationship between immigration and national security, the prevalence (or lack thereof) of immigrant communities, and the effects of immigrants on small towns in the United States. Still another factor that may be addressed would be American immigration policy, which has been notoriously difficult to enact into legislation.
By looking at some of these issues, you should be able to form a well-reasoned argument about immigration's effects on the United States.