Windows
Download and install TortoiseSVN.
Create a new folder for rAthena files.
Right-click the folder and choose SVN Checkout... from the menu.
In the Checkout dialog window, enter URL of repository: https://rathena.svn.sourceforge.net/svnroot/rathena/trunk/ and click OK button.
Checkout begins and a new window with list of files being downloaded appears.
After checkout is finished, close the window with OK button.