Aegisub is a 100% free, open source software for creating video subtitles.
To make sure you are getting the latest version of any software, I always recommend downloading from the official website. The download button will take you to the official site where you can download this program.
Aegisub can help you time subtitles to audio and includes other features and tool to style your subtitles. Watch this for help to install Aegisub.
Watch tutorials to learn how to use Aegisub.