If you want to set up your Yahoo! Mail account on your Windows Phone 7 mobile phone then follow the given instructions to do that.
Before proceeding further this is important to know that to make set up Yahoo email account on Windows Phone 7 mobile, your device must be connected to internet via Wi-Fi or with cellular data service of your carrier. See how to connect to WiFi Here, how to use PC / Mac internet on mobile Here / Here.
Set up Yahoo email on Windows Phone 7
On Start screen of your Windows Phone 7 tap the “Arrow” button or Flick left
Tap on “Settings” > “Email & accounts”
Then tap “Add an account” > “Yahoo! Mail”
Now, type “Yahoo Email address” and “Yahoo password”
Tap “Sign in” and all is done.
Your Yahoo Mail will get synced to your Windows Phone 7 mobile.