You will only be able to highlight changes from the point you shared the workbook. Also, you can only list new changes to a new sheet once the workbook has been saved.
To reiterate-
To share the workbook, click on Tool- Track Changes. Check the box at the top ('Allow changes by more than one user') and click on the second tab ('Advanced'). Change the number of days to keep the history to what you want and click OK. The workbook will then be saved. (Alternatively, just change the days you want the history to be kept then click back on the 'Editing' tab and uncheck the box and click OK. Select Tools- Protection- Protect and Share Workbook, which will allow you to give a password so that your workbook cannot be unshared).
To highlight or list changes made since you shared the workbook, click on Tools- Track Changes- Highlight Changes.
Also, check out the help files on shared workbooks as there are a number of things you can't do with them (pivot tables, edit macros etc.). Hope this is OK.