You have to pick a method or a standard for how your references, your sources, are to be arranged in your written assignments - no matter if you manage your references manually or use a reference management tool. These standards are often referred to as styles or output styles.
An output style defines which information to include in your references and your citations. It also defines how the information will present itself in your written assignment, both as to how your citations or footnotes will look and how your bibliography will look.
Which style to choose?
There are a lot of different styles and it can be hard to choose one. If you are in doubt it is always a good idea to ask the people you are writing your assignment to or the people you know will be reading or correcting your text. As a student make sure to check with your supervisor, or maybe your study handbook has a section with guidelines on reference management.