The easiest way to do this is to take the broken spring to a local garage door dealer or home improvement center so they can help you find an exact match.
How to install garage door extension springs.
The new spring s diameter length and wire size or gauge must be the same as your old spring.
Do not try to remove the extension spring that is not broken when the garage door is closed.
Granted garage door spring installation work requires time and attention so it is best to save the task for the afternoon of a free day.
How to replace a garage door extension spring.