If you filed your tax return electronically there is no physical signature on the tax return that was filed.
If someone like a loan company or school is asking for a signed copy of the return, then just print out the Form 1040/1040A/1040EZ sign it and give them the copy.
On a Form 1040 or Form 1040A the signature is on the second page of the form, at the bottom of the page, where it indicates Sign Here
On a Form 1040EZ the signature line is on the first (and only) page of the form, at the bottom of the page, where it indicates Sign Here
