Can you copyright a cover of a song? How?

Your question raises a number of issues. First, how do you know the "copyright has run out"? The duration of a copyright is among the most notoriously complicated terms to calculate. This is because the copyright term depends on many variables that have changed with the copyright laws over the years, e.g., who the author is (anonymous, corporation, person), the author's lifespan, whether the work is published or unpublished, where it was published, when it was first published, whether or not it was published with a valid copyright notice, whether the copyright was renewed or not, the country you are in, etc. In the US for a copyrightable work today, the term is life of the author plus 70 years. Back in 1790, the US copyright term was 14 years after publication, with an option for another 14 years. Significant changes to the copyright laws and consequently their terms of duration occurred in 1998, 1978, 1976 and 1909, among others. Second, in order to obtain a valid copyright for a work, such as a song, your work must be "original". Original means that the work must have been developed independently by its author, and there must have been some creativity involved in the creation. This begs the question, what is original about your cover of someone else's song? Thus, it is questionable whether you have any intellectual property in the cover song recording. Obviously, if the original song were still subject to someone's copyright you would have to seek a license from the copyright owner directly or obtain a mechanical license indirectly in order to legally sell and/or distribute your cover song. Assuming that the copyright for the original song really has expired, there should be no need for a license. You would certainly be the owner of such a cover song recording. You might even be able to obtain a copyright registration for your song. This is because there is no substantive review at the Copyright Office of whether or not your work is in fact copyrightable. However, the validity of your copyright (your IP), regardless of the registration, might be called into question (e.g., during litigation) because of the lack of originality of your cover based on someone else's original creative work/song. As always you should consult a Copyright Attorney to fully explore your situation and what you wish to achieve.

Yes. File an application.

The copyright to the score has expired but your cover of the song (assuming it has some unique features and is more than just a rote repeat of another band's version) will begin its copyright protection as soon as you record it. Music is considered to have two types of copyright: one for the original score and one for the individual performance.

You can copyright it as long as you introduce creative elements to it.

How do you know the copyright has run out? Contact the Copyright Clearance Center (www.copyright.com) in Danvers, MA to be sure. Check first.