So follow the 'I have forgotten my password' procedure and Yahoo will let you reset it. If you have forgotten your uncle's first name then he can't have been a very close uncle or you are not his nephew.
i have forgot yahoo passwrod and i also forgot first name of my favorite uncle name
Here's a quick thought then: ask one of your parents. He's their brother/brother-in-law. If you've managed to forget the first name of your favorite uncle then let's hope they haven't forgotten the name of someone they grew up with.
it sounds like you are trying to hack someones account, i hope not, this is not the place for hacks, do you know your email addy and password ? get yahoo to send the information to the email addy you signed up with. This is quite easy to solve if you are genuine