The problem with doing this is you're going to lose your formatting (i.e., font size, color, layout [if you've got it all spaced a certain way and whatnot). To do it this way though, open up the document in Word and click somewhere. Hold down the CTRL key and tap the "A" key (to select all). Then, hold down the CTRL key and tap the "C" key (to copy what is selected). Then go into your email that you're composing, and click inside where you type, and hold down the CTRL key and tap the "V" key (for paste).
A better way to do it, assuming they will take word file submissions, is simply to attach the file to the email. Depending on what email you use (i.e., something on your computer, like Outlook, or something online, like Yahoo!), each has different ways of attaching files. Since you're doing this on Yahoo, I'm going to assume you have a Yahoo! email address and can send emails through that. All you have to do is fill out the to/subject parts on the email, click on the Attach Files button, click on the Browse buttons next to an empty slot, navigate through your folders to where your resume is saved, and then click on Attach Files at the bottom. Then you can put in a message if you want, and hit Send, and your resume will be attached to the email, formatted as you had it in Word.