Zotero and Mendeley are two of the free citation managers that are commonly used in academic circles; we have guides about them and some of the librarians teach sessions on them.
Plagiarism and academic dishonesty go beyond simply cutting and pasting text from an article or book into your paper without attribution; they can include paraphrasing without attribution or citing sources that you have not consulted in your Works Cited list. The consequences of plagiarizing another's work can be very serious. Citing your sources is the first step to avoiding plagiarism.
Citation styles used in agriculture vary and may include those for a particular journal. As noted below, the American Society of Agronomy has its own style manual online. EndNote Basic supplies a number of styles that may be appropriate. Zotero and Mendeley are free resources that do similar work.
Many databases provided by the Libraries have a "Cite" feature built in. When you are looking at the detailed information for an item, look for a link or button for citing. If the link is available, you will see a screen which provides with a citation in your preferred format that you can cut and paste into your bibliography.