Plagiarism is all about using other people's work without acknowledging that work. The way to avoid plagiarising is to cite your sources, all your sources. Most colleges or universities will provide you with both a style guide for your papers and a document about plagiarism. Use them both because strictly speaking failure to reference your paper correctly, even if you do list all the sources, can be considered plagiarism.
An often ignored aspect of plagiarism is help people have given you in writing the paper. This is often ignored because it is hard to tell if, for example, someone has proof read your paper and given grammar or spelling corrections etc. Strictly speaking, I suppose, people should even acknowledge their spelling checkers although this is very rare indeed. It is much more common for people to cite dictionaries though it is for the same reason. Generally though this is unnecessary unless you have quoted or paraphrased a definition from the dictionary.
You must cite everything you used. People commonly think that it is enough to cite only the sources you quoted. This is not the case; if you paraphrase a point then this must be acknowledged; even if you just use an idea suggested in another document in a way that you don't think comprises a paraphrase, you must cite it.
I have been known to cite sources that I haven't quoted or paraphrased simply because they provided valuable background material that allowed me to understand the ideas in papers that I did quote or paraphrase. This is the kind of thing that would be considered using an idea from a source as suggested above.
There really is no reason or excuse for not referencing your paper properly. Papers that have long reference lists get better marks than those with short reference sections. Papers that follow the style guide get better marks than those that don't.
So, to sum up:
- Follow your institution's style and plagiarism guides especially the section on references and bibliography.
- Cite sources you have quoted.
- Cite sources you have paraphrased.
- Cite sources you have taken ideas from.
- Acknowledge all help you have received.
- Finally, enjoy writing those papers. If they aren't interesting to the author they won't be interesting to the reader and your marks are likely to suffer. If you can never manage this then maybe you're studying the wrong subject.
I forgot to mention before; if you are found guilty of plagiarism you will more than likely be thrown off your course, expelled from your institution and generally be dishonoured.
If you plagiarise here on everything your write-up will get nuked. See E2 FAQ: How to cite your sources.
On that note thanks to wertperch, jrn and TallRoo for their input.