want to lock only a specific group of cells
Cell Protection concerning different properties is set in the cell format. See tab ‘Cell Protection’ of the respective dialog.
Cell protection is, however disregarded as long as not the sheet is set protected.
You can set/change the sheet protection via the context menu of the sheet’s tab or going the menu path Tools > Protect Sheet.
The default cell style of the default template is set protected against editing. By default the protection on the sheet level is off, of course.