There are many sources on the Internet for LaTeX/TeX. However, I have always used the distributions provided by the Latex project portal. Here you can find distributions for Mac, Windows and Linux. These are ready to use distributions that will install LaTex/TeX in your computer with just two or three clicks. In addition to the main LaTeX/TeX programs, these distributions contain several other utilities like documentation files, spell checkers and LaTeX editors.
Another important source for LaTeX/TeX is the comprehensive TeX archive network (CTAN). CTAN is a central archive system containing a repository of packages (5535 when writing these lines) and TeX distributions. We will be coming a lot to CTAN pages, especially, when looking for documentation about LaTeX/TeX packages.
Finally, if you do not want to install anything on your computer you can still use LaTeX/TeX. There are several online portals that will allow you to typeset your documents with LaTeX/TeX. If you are interested in these options, take a look at ShareLaTeX or Overleaf.
As mentioned before, I use the LaTeX/TeX distribution provided by the LaTeX project. So far, I have only used the MacTeX distribution but the installation process for Windows is similar.
Installing LaTeX/TeX is very simple. Just follow these steps:
1.- Go to the LaTeX project portal and download the installation files
Once you are there, just scroll a little bit down until you see the names of the operating systems. Select your distribution. You will be taken to a new page from where you can download the files.
In the case of Windows, there are three alternatives: MiKTeX, proTeXt and TeX Live. I have never used any of them but according to the description of the packages, I would try them in this order: protTeXt, TeX Live and MiKTeX as the last choice.
2.- Unpack and install
If the downloaded file is a compressed file, double-click on it to unpack the file. Then, launch the installation process. I always left all the default options. Thus, read the on-screen options to make sure that everything makes sense and basically keep pressing next, next, install, finish.
Congratulations, now you can use LaTeX/TeX.