To Test of Not to Test...

TL;DR

As a relative beginner, should I add testing after the fact?

Just for some context here, I have pretty much a solid year of self-taught (read: laracasts and SO taught) development under my belt.

I've developed my first site/app for my employer and it's in production. At this stage, it's done, insofar as the site works like the customers and bosses want it to work, but of course, there's really no such thing as done.

I've made a few passes at extraction, DRYing it up, writing descriptive docblocs and such. There's still more to do, but it occurs to me as I watch many of the testing lessons, that perhaps I should work toward testability before continuing with the refactoring.

So I guess my questions comes down to

• Which of the various testing ecosystems I should tackle to learn and implement?
• Is there a different methodology or tool when adding testing after the fact?

crap, can't edit the title... ;)