You'll need an app that works for excel files. I use Documents to Go, others prefer Numbers (Apple's version of excel) and i'm sure there are other apps.
One thing to bear in mind, all apps are skinnied down versions of the main program, which means they all have a limited functionality. They will work, just don't expect full equations etc from them.
Other suggestions are good.
Try Splashtop 2 - Remote Desktop. If you're around wifi and have excel on your computer, this could work very well for you. If you have a cellular ipad you can connect for $1.00 a month or $10.00 a year. I suggest you google Splashtop and read tech media and user reviews. You'll find the app in the itunes app store.
Hope this helps.