the major feature of yahoo login page is to enter ur uid n
psswd and enter in to the website...
1.explore the yahoo page..
2.when the page is opened,place the cursor on the user id
textbox,and input the id.
3.now place the cursor in the password txtbox.
4.give the input or type the password.
5.press sign in
6.u'll be taken to the corresponding page.
7.if u don't have a uid,psswd already created,then
8.press the sign up button and fill all the details in it.
9.now login and try for your own mail page.

i think u have to write pre-requisite such as yahoo 
homepage is open or cursor is in login window.
1. then type yahoo id 
after it  cursor will set on password window
2. type ur password
3. click on "sign in"
if both r right ur login page should b open
else it gives msg"verify ur id or password"
if u have choose sign up option
it should open the sign up form and till u not fillup all 
mandotary fields as per rule it should not submit ur form.
and after creating ur account login process is same.
but i am quitely doubted about sign up option guys give ur 
opinion

I think as per the question to check for the major login 
functionality, following scenarios are to be tested:
1) Enter UID & PWD and click on Login/Sign using mouse.
2) Enter UID & PWD and hit enter button on keyboard.
3) Enter only UID or PWD and login.
4) Do not enter any field and hit login.
5) Enter one field with wrong information and try to login.

Correct me for any modifications.