Microsoft Excel is the industry standard - if you have that, you are all set in regards to this question!
If you don't have a spreadsheet program, there are a few options available to you: You can download one of the free spreadsheet programs available (OpenOffice Calc and Gnumeric are good free spreadsheets) or use the Google Docs spreadsheet program online. I would recommend downloading either OpenOffice Calc or Gnumeric, as those programs are faster than Google Docs and the interface on Google Docs is a quite a bit different than Excel.
I find Google Docs a good place to store small spreadsheets, but if I'm going to do some actual work on a spreadsheet I am doing it in Excel (or Gnumeric or OpenOffice Calc, both of which I use at times). Also, most jobs you might be in and/or classes you might take will be using Excel, so it is better to learn on the industry standard or something close to it. In my opinion, Gnumeric or OpenOffice are fine to learn on - the basic formulas are all the same, and if you are reading this you are a long way from using the more advanced Excel options at this point. Once you are comfortable with one of the free options, when you have to transition to Excel it will be pretty easy to do.
