Garage Door Spring Broken

I have a spring broken on my garage door. I've found a few tutorials online to replace the springs. Two questions:

1. Should I replace the non-broken spring too? I assume yes, but I know nothing about garage doors, so figured I would ask.
2. How do I find the correct spring to replace the old one(s)?

Here are the old springs & door:

 

Those are torsion springs, and yes, likely since one has failed, the other isn't far behind. Much easier to replace both at the same time.

That said, it's not usually considered a DIY project. They are under a LOT of stress and if you do something wrong, the energy can be released dangerously. I personally would recommend having a garage door person come out to replace them.

There are online tutorials if you feel comfortable doing it yourself. It's not complicated, but you do need to be careful, and get the correctly sized spring - they are usually color coded based on the strength.