To download the Cydia app store, go to the official website Cydia and download from the forums. You must have a Windows operating system since that is the only available operating system that Cydia uses.
The iTunes software program must already be installed on your computer prior to using Cydia. Do not download the program from any other website, as it might be a virus or tampered with to collect your personal data. Note that installing this software permits breaking Apple's warranty. The program is unstable and could cause your phone to become unworkable. This program also is not covered under manufacturer warranty, even if you bought the Apple extended warranty.