If you e-file your return, you do not need to send anything by mail. However, if you opted to file by mail, you will need to sign your tax return (Form 1040, 1040A, or 1040EZ) before mailing it. You should get an instruction sheet that lists what you need to attach and a mailing address.
And yes, you can mail it on the 17th. As long as it is postmarked by 04/18/2017, it is considered 'on-time'.
