Well, Monaco is a fixed-width font, so there's no way to make Sublime display a non-fixed-width font "like Monaco." =D
But! To change the font settings, just go to Preferences > User File Preferences and add the "font_face" (and "font_size" if you want) key[s]. Once you're done, it should look something like this:
"color_scheme": "Packages/Color Scheme - Default/Railscasts.tmTheme", //something like this might already be at the top of the file, don't change it!
"font_face": "Helvetica Neue", //your font name here
"font_size": 14 //your font size here
EDIT: If you want to tweak other stuff, you can view the default settings from Preferences > Default File Preferences. Just remember to make your changes in the User File Preferences file.