If you're just starting out and you haven't gotten the hang of spreadsheets yet, here's something to help!
This spreadsheet assumes that your e-books are priced at $2.99. The royalty estimates for Amazon and Smashwords will give you a slightly high estimate - there are fields to fill in actual royalties for the month. It will highlight the highest sellers/earners, per-site and overall. It includes a sheet for each month, plus a yearly total sheet that will sum up the other 12.
It's an .ods file, which will work with OpenOffice. It's free - if you don't have a certain other office software suite, you should go download it!
Again, this spreadsheet is certainly not perfect, but it could be a good starting point. Off the top of my head it contains examples of (fairly simple) functions and conditional formatting.
Good luck! :)