Application on at power up

In El Capitan, the options are to have the startup items shown when the user logs in by checking or unchecking the box in the column marked Hide for the item. Click on the + below the listing window to add an item. Click the - to remove an item from the listing. Supposedly, you should unlock this to make a change, but I added an application to the items without having to unlock.

iTunes has been one of the items that starts every time my Mac Pro is turned on, and I really had not taken the time to determine why before now.

In High Sierra it is in Apple menu > System Preferences > Users & Groups > [username] > Login Items (near the top of the Preference). I think it was the same in El Capitan.

That is in the Users&Groups-LoginItems window. Preferably none or only a few entries in there.

Look also in StartupDisk->Library->StartupItems: here there should be none.