Some OS X app has the option in its Preferences to automatically start itself when you login. This is convenient for those apps that you must run at all times whenever you’re on your Mac. Take the Preferences for the app Fantastical as an example, the option is under the check box “Open automatically At login”.
For app that does not offer the option to start at login, you can set it up from its dock menu. Right click on the app icon on the dock to reveal its dock menu. Select the option Options > Open at Login.
You can also set up login items via System Preferences > Users & Groups under Login Items tab.
Click on the ‘+’ button to add an app to start at login. Or use the ‘-‘ button to remove app from opening automatically when you log in.