Citations are a key component of Academic writing. We sure stand on the shoulders of giants but it is key to give credit where credit is due. Lack of or improper citing can result in plagiarism, which is a (very) serious matter. In this page, you will find some basic reference on how to cite and a few links to different tools that can help you cite properly without much hassle.
Citation essentially implies to attribute bits of knowledge you have borrowed from other sources to their original author, within the text you are writing. There are two basic aspects to understand about citation: one is conceptual (why and what to do) and the other one is technical (how to specifically do it).
Conceptually, citing implies recognizing who produced or pushed forward the idea you are using in your manuscript that is not yours. This means that, whenever you mention a concept, result or any bit of knowledge you have learnt from somewhere (i.e. it is not of your authorship), you need to properly express where it comes from. This is usually done in two parts:
Depending on the nature of the manuscript you are preparing, you will have different requirements as to how to present this, so make sure you know the rules by which you have to play. In any case, two general principles apply always: consistency and full matching. Being consistent implies to pick one way of mentioning publications and using it throughout the document. A full matching means that each and every of your citations mentioned in the text has a reference in the references list, and that every single publication mentioned in the references list appears in the text at least once.
Have a look at the following two resources to dig further on how to cite properly:
As mentioned in the Student Handbook, GEES has a general rule to, unless stated differently, adopt the Havard System.
The technical part of referencing is much simpler (not neccesarily easier). There are many options as to how you can insert citations (see this WikiPedia list) or manage your references in a paper, from entering them manually to fully automated systems of relative complexity. At the end of the day, it all boils down to a personal choice, play with a few of the options and find the one that best fits your needs and preferences. But remember, the most efficient route in the middle run is not neccessarily the fastest one to learn or the quickest one the first time you use it, so be patient and give yourself time to get proficient with your choice.
General popular options:
Options specifically designed for particular typesetting systems:
For a more comprehensive list, have a look at the WikiPedia list of reference management software.