Anyone who can help?

Okay, I've never copped to being a computer person, but I've learned a lot through blogging. Although I admit, I still have PLENTY to learn. Plenty. Like this:

How do I change the font on my posts and titles? I downloaded a cute "Will and Grace" font, and it's installed, but when I click on the font button on the posting page, it's not there. Just the same old ones. FYI, for any helpers out there, I use Firefox, and an iMac.