Stopping Unwanted Apps in Dock at login

To start, in System Preferences => Users & Groups, click the padlock in the lower left and enter an administrator name and password, then look in Login Items tab. You can remove programs that are there.

Also, after starting up, go to each program that opened automatically and open its preferences and look for a setting to have it start when the computer starts up; you can disable those options within each program so it doesn't automatically start up.

Which programs are starting automatically? Some may require more aggressive measures to squelch.

It depends on the application.

In most cases, you can Control-Click the icon in the Dock, select options and then ensure open at login is NOT checked.