Notes only offers three default fonts, and there is no GUI option to change these or add others. You can, however, add fonts by editing a plist file.
Open /Applications/Notes.app/Contents/Resources/en.lproj/DefaultFonts.plist (or the equivalent for the language you use), and you'll see three sets of text like this:
<dict>
<key>FontName</key>
<string>Noteworthy-Light</string>
<key>Size</key>
<integer>15</integer>
</dict>
Copy one of them, paste it at the below those three sections, and add your preferred font and size. Note that you can specify the weight with "-Light," "-Bold," etc. Save the file, relaunch Notes, and choose your new default font from Format > Font > Default Font.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20120811055524908