If you’re new to the whole concept of spreadsheet programs, check out my new course, published by Tuts+: Intro to Spreadsheets. It’s not Excel, only!
Of course, Excel is the 800 lb. gorilla in this category, and most of the course focuses on it, but the course will also help if you’re using other applications, like Google Sheets, Apple Numbers, LibreOffice Calc, and the web and mobile versions of Excel. You can finish the course in about a day. Exercise files are included.