The shortcut keys appear in button names in the button bar (right side of the Tally screen). You can either click the button from the button bar or press the relevant function key or character underlined/double-underlined.
The buttons have a function key before the button names ( Eg: F1: Select Cmp) which means you need to press F1 key (Function Key) to select the 'Select Company' screen
The buttons have an underlined character ( Eg: F3:Cmp Info), which means you need to press ALT + F3 to select the 'Company Info' screen.
Some buttons have a double-underlined character (Eg: As Voucher) which means you need to press CTRL + V to select the 'Voucher' in voucher mode.
The shortcut keys available in Tally are listed in the below slide show: