First, go to the Yahoo! registration page. Then, fill in the blanks with your name, gender, country and zip code. After that, choose a new ID and a strong password. Then, you have to insert an alternative email address, for safety reasons. Finally, you have to submit.
It is preferable to choose a strong password for the safety of your data. It is important to be really careful when you type your personal data, because it's most likely that you will need it correctly written, especially if you ever need to prove that the account is yours. The safety questions have the same purpose.