We've all seen ugly documents. They are everywhere. Although you may not realize it, many times a document is hard to read or unprofessional looking because of the font choices the designer made. Although there is no "right way" to select fonts, if you aren't confident in your ability to choose fonts, here are a few guidelines that may help.
1. Choose a font that matches the tone and audience of your document. For example, a gardening newsletter will have a different readership and "feel" than a data sheet for a high-tech product. The gardening newsletter might use a more fun and light serif font, whereas the data sheet might use a more austere sans-serif font.
2. Use contrasting font styles for body text and headlines. For example, you might use a bold sans-serif heading font with a plain serif body text font. You generally don't want to mix two very similar typefaces. They won't provide enough contrast, so it's more difficult for the eye to pick out the headlines from the rest of the text.
3. Avoid using too many fonts. This problem is known as the "ransom note look" and almost invariably makes a document hard to read. However, with that said, sometimes using a number of different fonts can be an interesting effect in an advertisement or other very short document. You definitely want to avoid ransom note-itis in a long document like a report, however.
4. Be consistent. If your headings are set in a particular font, don't switch it midway through a document, unless you have a good reason for it.
5. Avoid hard to read fonts. Some decorative fonts are designed to only be used for headlines or even just drop caps. If you can't read the type, rest assured no one else will bother.
6. For short documents or logos, try out many different fonts. If a short phrase is a focal point, experiment and observe how the letters relate to each other. When you really look, it's amazing how different the same set of words can appear when set in different typefaces.
These hints may help, but if you really feel you have no aesthetic eye for these things, find someone who does and get their opinion. You'll be glad you did!