How can I legally sing a cover song and post on YouTube if all I have is my voice? ?

All the other answers are only partly right.

No, you cannot post someone else's song without permission. That much is very clear. 

But you can buy a licence at places such as TuneLicensing (there are other ones out there, just Google "cover song licences"). It's a fairly straightforward process these days. 

But what happens if you don't licence the song beforehand? Although you are committing a copyright infringement, it is likely that your video would be taken down before legal action would be undertaken. YouTube are fairly good at spotting music in videos and removing what is not licenced. 

So your time, effort and perhaps money would have been for nothing. 

What do you mean by cover song? Do you mean if you cover a song? If you're using a song to make money you need to get permission or you can be charged with copyright infringement.

You've got to get permission from the copyright holders for that.  If you make money performing someone else's music (and it will be copyrighted if it's popular), you've got to pay royalties to the owners.  They'll come after you in court if you don't ask first, and they'll win.

You can't legally post yourself singing a song that you don’t own the copyright to unless it's either in the public domain or you have the permission of the copyright holder.

You can obtain backing tracks to sing over from many sources, just Google, backing tracks for singers. 

This would be perfectly legal but, again, you would be breaking the law you post them on You Tube.

If you want to LEGALLY sing/monetize a copyright song then you need permission from the copyright holder. 

Say you wanted to cover Crazy in Love - you would need to contact ASCAP about obtaining a license. Once you've negotiated that - it's usually a royalty payment, so a percentage of profits - then you need to get your backing music. The best way to do that is to record it yourself, which will mean paying musicians. If you can't do that then you need to find an existing backing track - and pay a fee for that as well. That may be a simply flat fee, or another royalty.

That is assuming you can even obtain a license - some artists refuse to grant any, no matter how good a musician you are.

As long as the lyrics are in the public domain, it's legal.  Any song recorded or published in the US before 1923 would qualify.  Anything after that will require some research.  You may find some help at https://www.pdinfo.com/public-domain-information-p...