Here's the short list of the best shortcut keys to know. If you learn only these key combinations, you're guaranteed to save time.
1. Ctrl+Z Undo
2. Ctrl+C Enter, Ctrl+V Ctrl+X Copy, Paste, Multiple Paste,Cut
3. Ctrl+F, Ctrl+H Find, Find & Replace
4. Ctrl+P, Ctrl+S, Ctrl+F4, Alt+F4 Print, Save, Close, Close Excel
5. Ctrl+Arrow Move to edge of region
6. Ctrl+* Select current region
7. Ctrl+A Select all cells
8. Ctrl+HomeCtrl+End Select A1, Select last cell in used range
9. Ctrl+Shift+End Select from active cell to last cell in used range
10. Ctrl+Shift+Home Select from active cell to A1
11. Ctrl+Page Down Ctrl+Page Up Move to the next sheet, Move to the previous sheet
12. Ctrl+Tab Move to next open workbook
13. Ctrl+N Open new workbook
14. Shift+F11 Insert new worksheet
15. Shift+F3 Paste function window
16. =+FunctionName+Ctrl+A Insert new function
17. Alt+F11 Open VBE
18. Ctrl+Shift+Enter Array formula
19. Ctrl+F3, F3 Define name, Paste name
20. Ctrl+Spacebar Shift+Spacebar Select columns, Select rows
21. Ctrl+1, Ctrl+B, Ctrl+U Format cells, Bold, Underline
22. Ctrl+; , Ctrl+shift+: Current date, Current time