Introduction to LaTeX
LaTeX is a typesetting language for creating scientific documents, notes, and papers. It is the standard tool for writing papers in physics, mathematics, computer science, and statistics and is commonly used in other fields. It allows one to easily write equations, add figures and tables, and organize citations for documents. It can also be useful for taking notes, writing final reports and writing-up problem sets.
There are two main ways to use LaTeX: through an online website or locally on your computer. A number of sites exist (Overleaf and Authorea are two I've used) which make it easy to create and edit LaTeX documents. These sites are very useful for collaborating on documents in real-time, but may have limitations in terms of the number of private documents you can create and may not be able to use some LaTeX packages (although they are compatible with all typical packages).
If you want to use LaTeX locally on your computer (and once you become proficient, you'll probably want to be able to) you can download and install a LaTeX distribution. The latex-project.org site has good resources for all operating systems. Most LaTeX distributions come with an easy-to-use, LaTeX aware editor.
I led a LaTeX workshop with CalNERDS in the Fall of 2016. The workshop outline which includes links to resources can be found here. An Overleaf document that has a bunch of examples of different LaTeX functionalities is here.