Apple's office suite of applications is named iWork. The iWork suite comes with a word processor named Pages. If you wish to capitalize the first letter of every sentence in Pages, there is a preference that enables this. Auto-capitalization is not enabled by default, but it can easily be turned on by selecting a check-box in the application preferences.
Open the Pages application on your Mac. Pages is installed in the iWork folder located within your system's Applications folder. Navigate with Finder and double-click on Pages to open it.
Open the Pages Preferences pane by selecting the "Pages" menu on your menubar and then "Preferences" from the top of the drop-down menu.
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Choose the "Auto-Correction" tab within the Pages Preferences pane. Click on the "Auto-Correction" icon to select this tab.
Check the box next to the "Fix capitalization" option and close your preferences by clicking the red button in the upper left corner of the window.