I forgot my secret questions answer of my yahoo mail............what can i do?

You will need to contact Yahoo customer care directly. Go here:

https://io.help.yahoo.com/contact/index?page=home&locale=en_US

Select Yahoo Account
Under Topic, select Hacked accounts
Under Sub-topic, select Unable to verify my secret question
Under Recommended options, skip the Resolution articles and click Call Us

Remember them - no-one is allowed to tell you what they were, and without them you are not going to get back into that account. You may just have to abandon it and open a new one.