When you record a macro (Tools>Macro>Record New Macro) you have the option of assigning a shortcut key. A shortcut key allows you to run the macro with a key combination, such as Control+Shift+i. To change an existing shortcut or to create a shortcut for a macro that doesn’t have one, start by opening the Macro dialog (Alt+F8)
Click on the Options button and enter the desired shortcut.
In this example, I entered a capital S. Beware that existing Excel functions have shortcuts assigned to them. Had I entered a lower case ‘s’, Control+S would have been the shortcut and would no longer be the shortcut to Save the active workbook.