The sources you use must of course be relevant for your topic, but they should also meet other requirements. You must consider the nature of the type of information being cited as well as the context in which you are writing, since this will determine what sources are considered legitimate to use.
The most important thing to remember when citing is that you should always refer to the primary source, which is the first source where the thing you want to cite occurred (see Primary vs secondary sources). When referring to research results, you should cite the first peer-reviewed publication of the research. This is usually an original research articles or monograph, but it might also be a conference proceeding.
As a general rule, it can be said that traditional scientific sources (see Scientific sources) are still the most commonly used once in Science. The main sources are research articles and review articles published in peer-reviewed academic journals, followed by books and chapters from edited books. However, the various disciplines all have their own conventions regarding what is considered a relevant source, and even within departments there may be differences between subjects. The theoretical disciplines and subjects will primarily favor scientific sources, whereas the more applied ones tend to make use of a wider set of sources including for example patents, legislation and reports from public authorities.
If you are writing a report, essays or other assignments at the university, you should always follow the instructions given by your teacher who is an expert on the conventions within their specific subject.