If you are using Windows, then git can be downloaded from the git website. Be sure the select the 64-bit standalone installer. There are a lot of options to choose from when running the installer but it is okay to use the options that are selected by default. Again, it is important to ensure that git is added to the PATH environment so make sure the appropriate option is selected (see below).
If you use OSX, then to install git, just open a terminal type git and you should be offered to "install the OSX command line tools". Follow the instructions and you should have git installed.
Simply download from the website and install.
Once you have everything installed, please check that it works correctly.