Many people feel that immigrants are taking jobs they want. While it is true that many immigrants take jobs Americans don't seem to be taking, more and more Americans are willing to take them now. There still are fewer non-immigrants working as house-cleaners and farm workers, but there are plenty of jobs held by illegal immigrants that Americans do want. For example, there is a large number of illegal immigrants in construction. Many construction jobs actually pay well, and these are jobs that non-immigrants could take for fair wages if illegal immigrants weren't working in them for less.
Immigration helps the US economy in a couple of major ways.
First, immigration means that the US has a workforce that is not declining in number. This is a very serious concern for other rich countries. Japan, for example, is aging rapidly and will have too few workers to provide for the elderly. With immigration, America does not suffer from this problem.
Second, immigrants often fill jobs that require lower skills and education. This frees up "native" Americans to take better jobs. If we do not have to have a lot of "native" Americans doing low-skilled jobs, they can get more education and can take jobs that add more value to the economy and which give them better pay.