Document any specific ideas, opinions, and facts that are not your own. The only thing you don't have to document is common knowledge.
For example: you DO have to document the fact that 103 cities in New York State were originally settled by English settlers, since this is a specific fact that is not common knowledge. You do NOT have to document the INFORMATION that New York State has places named for English cities, since this IS common knowledge.
There are two categories of common knowledge, information that's known to the general public and information that is agreed upon by most people in a professional field. Yet sometimes common knowledge can be tricky to define. A good rule is if in doubt, document.
The rule of thumb is that whenever you use information from sources you should comment on the information. Your comment should be approximately the same length as the source itself.
Citing sources at the end of the paper is easy. You merely put your notecards with the source information on them in alphabetical order according to the authors' last names, and then follow the correct format for providing the essential source information.
The good news about documenting your sources within the text of your paper is that footnotes are out of date. Today most research papers put the basic source information inside parentheses right in the text of the paper. The parentheses comes at the end of the sentence, or group of sentences, that contain the source's information.
Merely documenting paraphrases and summaries at the end of paragraphs leaves your reader confused. Does the documentation refer to the last sentence? the whole paragraph? part of a paragraph? So you also need to show where the source's information starts as well as ends. The easiest way to do this is to use a phrase such as "According to Dr. James Watts. . ." or "Carly Simon maintains that. . . ."
Plagiarism is a serious offense within the academic community. You plagiarize whether you intend to or not when you don't credit others' ideas within and at the end of your paper. Remember, even though you may have rewritten ideas and information using your own words in a paraphrase or summary, the ideas and information are not yours. You must cite your source.
More information on plagiarism from Princeton University
How to document sources