Updated June 5th, 2019.
Normally, when you click on the battery menu you get basic information about your laptop’s battery, like this:
Turns out you can get more information. Press and hold the Option key, then go to the battery menu again. You get this:
If the condition says “Replace soon” it’s probably time for a visit to the Apple Store. You can get a battery from Amazon and do it yourself, for certain models– otherwise, go to Apple. Figure about $129 for a battery at Apple, installed. Batteries do eventually wear out so if yours says something other than “Condition: Normal” that’s just the way it goes.