Using LaTeX requires a TeX distribution, which enables the compiling of .tex files into readable documents, and a text editor to create the .tex file itself.
Because of the sheer volume of packages available for LaTeX and difficulties translating them to an HTML environment, browser-based options are less flexible but more accessible, with the added benefit that they allow collaboration on documents.
Many freely-available LaTeX editors are available. An extensive list of editors is linked to below along with a few specific suggestions.
Note that free text editors, such as SublimeText or NotePad++, often include preset .tex formatting.