FreeOCR is a well-respected freebie for extracting text from scans. (It might not be as flexible as some of the commercial products which are available but, with Omnipage Ultimate retailing for $450, you probably won't want to fork out for such software anyway):
http://www.paperfile.net/
The instructions are straightforward:
http://www.paperfile.net/help.html
The online services suggested by Tuvok require you to scan your document to an image file, or to a pdf file first. (All three of those site will work with Jpeg files. The first two can handle PDF files too but the last one can't). You Click the 'Upload' or 'Browse' button on one of those sites, navigate to the location of the relevant file on your hard drive and double-click on it. You then select the output format you require and click 'Convert'. Once the conversion is completed you'll be presented with a download link, enabling you to access the file containing the text.