On your HTC phone go to your Home page.
Slide towards the Mail tab.
From the viewed options tap on New Account (in few cell phones you will have to tap menu and then enter into New Account).
Another screen will come into view showing different email providers. You will have to tap on Yahoo Mail, but if Yahoo is not available then tap on "Others."
In the subsequent screen you will be asked to add the below given information:
Enter your Yahoo Email Address
Enter Yahoo Email Address Password
Then tap Next.
You will be asked to enter your name and accounts name, which will appear on your phone.
In the end tap Next
In case Yahoo account type is not present on your HTC phone then you will be asked to enter a few more details which are related to your ingoing and outgoing server information; this data can be acquired from the Yahoo service providers.
Next you will be asked to select the frequency with which your phone will automatically send and receive emails.
In the end tap Done to finish the procedure.