Computer doesn’t detect your Android device

First

Plug in your device via USB if you haven’t.
Make sure your computer hasn’t popped up a window to ask if it can connect to your device. If you haven’t allowed it, that’s the problem.

Second

Run adb devices in a terminal window on your computer. This will list all devices it’s able to connect to. If it shows up, you’re good to go.

Third (If necessary)

Turn on developer options

1. Scroll down and tap About phone on your android device
   (sometimes About device).
2. Find Build number.
   (It might be under a submenu like: Software Information or Version)
3. Tap the build number repeatedly until it says you’re a developer.
   (About 10 times).

Enable USB debugging

1. Go back to Settings.
2. Select System / Developer options
   (on some phones, it’s under Additional settings or directly under Settings).
3. Enable check box for USB debugging.

Restart ADB (If necessary)

If you run adb devices again and it’s still not appearing check you haven’t missed a permissions popup on your computer again, first. If not try restarting ADB:

adb kill-server
adb start-server