If you are already a Yahoo! Music member, you will probably be signed in automatically when you visit Yahoo! Music. This is because Yahoo! Music uses cookies to recognize its members.
However, if you share a computer with other people or you are visiting Yahoo! Music from a different computer than you normally use, you may have to sign in.
To do this, go to the Yahoo! Music home page and, at the upper right, you will see a "Sign In" link. Enter your Yahoo! ID and password and click "Sign In." If your sign-in is successful, you should see the Yahoo! Music home page with "Hi (your Yahoo! ID)" displayed in the upper-right corner.