Someone has managed to get into your yahoo account and is spamming your friends.
1st Thing: Change Yahoo Password
Click Here https://edit.yahoo.com/config/change_pw?
2nd: Be sure to have your computer checked for virus or key-loggers
3rd: Check your account again. Sometimes people who have gained access to your email account will set themselves up to receive email notifications of password changes. Then they login again and change your password to something else.