In your rocket launcher menu of applications you can right click the application icon and click Add to startup(A) or Remove from startup(R)

If you want to start an app on boot, you can create your own .desktop file :

Open terminal
Type:
sudo gedit ~/.config/autostart/name_of_your_app.desktop

Put this in your .desktop file :
[Desktop Entry]
Type=Application
Name= name_of_the_app
Comment= comment_about_your_app
Exec= command_to_execute

Example with thunderbird:

sudo gedit ~/.config/autostart/thunderbird.desktop

thunderbird.desktop file:
[Desktop Entry]
Type=Application
Name= Thunderbird
Comment= get my mails
Exec= thunderbird

If you don't know the command, type this in your terminal:
compgen -c | grep app_you_are_looking_for

If you can't find your app with compgen but you have the icon in the launchpad (like Google Hangouts), try this:
-> Open your launchpad
-> Right click on the icon
-> Select "send to the desktop"
-> Right click on the icon in the desktop
-> select "properties"
-> Copy the command