Most often, you will need primary sources. Primary sources are written by those who worked on the experiments. They directly report the findings of research or studies and give information about how experiments were conducted. 

You can find primary sources in several ways. One is to read the references lists of reviews or book chapters. Review articles and books are usually secondary sources--they report, describe, or analyze the findings from primary sources. Sometimes it helps to read these secondary sources first to learn the background on a topic. When you find a citation ( a primary source) in a review article, you can use Google Scholar or the Library Find-It Box to locate the full text. 

Even when you are already reading a primary source, examine the references list to see if there are others that would be helpful for you. In some cases, older literature is just as useful as recent scholarship. 

You can also search library databases or sites like Google Scholar using keywords to find primary literature on specific topics. Sometimes this takes a little trial-and-error. 

In writing for biology, sources are often used in introduction and background (literature review) sections. In these sections, you might need to provide definitions, a historical description of how the topic has developed, or explanations of important work on the topic. 

If you're writing a lab report and you based your hypothesis or methods on previous studies, you should cite those studies in order to give credit to those authors. 

Most science writers do not use direct quotes from their sources; instead, paraphrase and given appropriate citations for the idea or concept. 

Use keywords. Keywords are single words or entire phrases that relate to the information you are trying to find. 

For example, in A Short Guide to Writing about Biology, Pechenik (2016) suggests that if you are looking to study the respiratory systems of reptiles, you might try words like: respiration, reptile, "respiratory system,"  lung, or the names of specific reptiles. However, you might also have to search using words like physiology or "comparative physiology." 

Search Tips: 

Sometimes the keyword that seems most obvious to you is not the one that brings up the most relevant results. If you are using all the keywords you can think of but aren't getting relevant results, try a quick Google Search to find more keywords to try. 

Keep in mind that it can be useful to combine keywords. If you are trying to find information about reptile respiration, it is not enough to search for only respiration or only reptiles; you need both. 

You can also search for synonyms simultaneously by using a capital OR between the keywords. For example, you can search reptile OR lizard OR iguana, which will return any item that has one of those words in it. 

If some of your keywords are more than one word or are an entire phrase (like "comparative physiology" or "Komodo dragon"), use quotation marks to hold them together in your search. This informs the search engine that you are looking for those words together, not interrupted by other words. Without the quotation marks, the database searches for any item that has the word 'comparative' somewhere in it and 'physiology' somewhere in it, but doesn't guarantee that the phrase "comparative physiology" ever appears. 

Search for word variations. If you want to search for reptile OR reptiles OR reptilian, it might be easier to search reptil*. Using an asterisk at the end of a word fragment like this is called a truncation mark. It will search for all keywords that are identical in spelling  up to the asterisk. If you searched reptile*, you would search for reptile and reptiles, but not reptilian. 

Practice click restraint. Once you've entered your keywords, don't click on the first result that appears--instead, scroll through one or two pages to get a general idea of what your keywords returned. Then you will have a better idea of if your search "worked" or if it would be beneficial to modify it. 

Use filters efficiently. Most databases will let you sort your results by time (newest first) or relevance (how many times your keywords appear). You can also filter out results in a certain date range or select a specific article type (which allows you to remove secondary sources or things like book reviews). Be careful, though--too many filters can confuse you and make it hard to remember exactly how you're searching. 

Consider taking notes of your search strategies. Where are you searching? What words did you use? What filters did you apply? Did the search 'work'? These notes can help you if you need to go back to find additional information.