To do so, go to System Preferences > Users & Groups and click your name on the left under Current User. You can remove login items one at a time and start up your Mac after each removal to see if the problem is gone. With a bit of trial and error, you can identify which app is the problem child. If you find yourself staring at a blue screen when you start up your Mac, it might mean that one of your startup items - apps that start automatically when you start up your Mac - is incompatible with MacOS. To leave Safe Mode and start up your Mac per usual, just restart your Mac without holding any keys. It may take a few minutes before you get to the login screen as MacOS runs its diagnostics on your hard disk. You can release the Shift key when the Apple logo disappears and the login screen appears. The Apple logo will appear and then the login screen.
To start up in safe mode, start your Mac and then press and hold the Shift key. In Safe Mode, MacOS will boot with the bare minimum of software and drivers required and will run a check of your startup disk and repair any directory issues that might be the cause of your startup ills. If your Mac fails to boot properly and you find yourself staring at a blank screen or gray startup screen instead of your desktop, then it's time to try booting in Safe Mode.
