'View' > 'Toolbars' > 'Forms'
In the box that appears, click 'Button' (second down on the right) then
click in the worksheet.
In the box that appears click 'New'
Underneath 'Sub Botton1_Click' enter:-
Range("A1") = Range("A1") + 1
Press Alt and F11 to close the window.
Change A1 to suit and you can rename the button by right clicking it.
After creating you command button assign this code. You might also consider
doing some error checking because it the cell contains text adding 1 will
give an error.
Sub Button1_Click()
Range("A1").Value = Range("A1").Value + 1
End Sub