Do you mean handwriting or performing "OCR" of a scanned document?
If the former, handwriting recognition in OneNote requires that you have a
Tablet PC. On a traditional laptop or desktop, handwriting recognition is
not available.
If the latter, there is likely some kind of OCR software that came with
your scanner. That's your best bet. Scan the doc, perform OCR, and then
insert both the scanned document and/or the text into OneNote.