Got to the Google analytics and add a property for your Website. After that you will get an tracking code.
Now go to Code Injection page in your Ghost admin and add that tracking code in the foot section.
Why doesn't Ghost have analytics built-in?
Because building analytics is an incredibly complicated thing to do, and even more complicated to do accurately, so we would really be fighting a losing battle. Google have tens of thousands of engineers, their product is completely free, and it's extremely good. We have 5 engineers, could not afford to build a secondary product this complex, and if we did - it would be demonstrably less-good than Google Analytics. So unfortunately the economics just don't make sense.
This is actually a good thing for users, though. When products have analytics built-in they're unanimously inaccurate and deceptive. You never know where the numbers are coming from or if you can trust them. Using a proper analytics service is a much better choice all round!