(I'll assume that you're using a Windows PC or laptop, rather than something like an Android tablet):
If you're using the Yahoo email website (rather than accessing the account through an email client, such as Outlook Express, Windows Live Mail or Thunderbird) I doubt that you can change the font you see on screen. It's the same as typing a post here on AB; you're stuck with whatever the website gives you. Of course you can make everything larger (by pressing Crtl and + together) but it will still be the same font.
However if text isn't looking clear on your computer there are two things you should check:
Firstly, go here and check that you can see all of the numbers, from 0 to 10, on both of the relevant lines:
http://reeddesign.co.uk/testcard.htm . (If not, follow the instructions to adjust your monitor).
Then ensure that ClearType is turned on and optimised. (Click 'Start'. Type 'cleartype' in the box. Click on 'Adjust ClearType Text' and follow the instructions).