To being a part of yahoo site system you should first have to create your own account in yahoo app. By following the steps below you will have the answer to the question how to create a yahoo account?
Go to www.yahoo.com and click the “sign in”.
Sign in page will be opened. Click on the “create account”.
Enter your phone number for sending of the verification code. As they send you the verification code. Enter that verification code on the page.
As they receive the verification code your account on yahoo is created.