Is is realistic to hope to cover (record) one of Joni Mitchell's songs
if you are a new musician?  What are the chances?

In the United States, once an artist has commercially released a song,
anyone may "cover" the song and re-release it, as long as it isn't
substantially altered in it's arrangement(to the point where it's
unrecognizable as a cover.)
In order to substantially change an original song that you want to
cover, you need permission from the copyright holder and pay whatever
royalties are required. This information (including each publisher's
address) is available through the following Performance Rights
organizations:

BMI (www.bmi.com)
ASCAP (www.ascap.com)
SESAC (www.sesac.com)

I hope this helps!
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