You have to pay Yahoo for the privilege of being able to forward your email. No joke. You need a premium account on yahoo to enable email forwarding.
However, you still may be able to do it through gmail, because GMail "checks" your yahoo mail technically, I think.
Try it out and let us know how it goes. If you don't know how to do it, go to gmail on your computer --> setting (top of page)-->account and imports-->add POP3 email address. Pretty straight forward from there. It works perfectly for my AOL email and my school email, but I don't know about yahoo. Someone will probably call me out and say I'm totally wrong lol.