You don't need to sign the form, if you are applying online.
Just fill the form, validate it, save it and then upload it. Once you uploaded all your files/attachments then click next, it will ask you your first and last name and one security question just to confirm it. That will be your sign and your done with signature.
Check this link as well:
http://www.cic.gc.ca/english/helpcentre/answer.asp?q=740&t=23
