Find your css or style.css or stylesheet file, in the Admin section "Presentation" / Themes Themes Editor - pick Themes Editor. Any well-made theme will have as its first file the Stylesheet file.
Since we know from your complaint that there's only one font operational for your blog, then it's usually true that:
The font is declared in the first block of the Stylesheet, in code that looks like this:
background:#e6e6e6 url(images/bodybg.png) repeat-x;
Look for a line similar to the one above that says `font:76% tahoma, verdana, sans-serif;'
At this point, supposing that you see roughly what I see when I look at my Stylesheet, it is an eminently intelligent, swift, smart, good idea to make a backup copy of your theme, by finding the WP-Content folder in your WordPress installation, opening it, and finding the Themes directory, then opening that and locating the name of your them. Then the ftp program that has shown you this will allow you to "Download" to your own computer, maybe in a file with your blog name--on your Desktop, not your Documents file.
Now, back to the stylesheet:
Don't change the size of 76% in that line declaring the fonts, or you'll be in worse trouble than you are now. In the following part of the line you see the names of two or more fonts. In the theme I'm using as an example, there are two font names, and then a font type, which is san-serif.
In HTML (of which CSS is a part) this is a way of letting a browser find a font to use, if neither of the first two fonts is available.
The simplest fix is to pick the fonts you want and put them in the place of the ones that your Stylesheet is declaring.
Then scroll down to the bottom of the window and look for the button outside the window that says "Update File."
Your Stylesheet will be updated with the new fonts.
Anything more than the simplest fix such as this, and you need to be working with a CSS manual handy.