You have to install the appropriate USB Driver. You may follow the instructions on this official link Install the appropriate USB Driver.
1) Things are simplified for Mac Users. Connect your android phone to Mac using USB cable.
2) In your android phone, Go to Settings > About Phone > Software information > Build Number. Tap Build number 7 times to eanble Developers menu.
3) Return to the previous screen to find Developer options near the bottom.
4) Turn on Developer Options and scroll down to enable USB Debugging.
In android studio, select app module in project window and Run Button in toolbar . Select your connected device and Ok. App will be installed on your phone and starts running.
Emulator is just a virtual device which android studio provides with the help of which you can run your apps without any need of real android device.
In android studio, select app module in project window and Run Button in toolbar.
2) Create a new virtual device.
3) In hardware menu , select the device you wish and Click Next.
4) In the System Image screen, select the version with the highest API level. If you don't have that version installed, a Download link is shown, so click that and complete the download.
5) Next > Verify Configuration of AVD(Android Virtual Device) > Finish.
Select device on which you want to run your app > Done.