A common design choice is to uppercase all letters in the top navigation menu.
You can easily do this on your site by adding this custom CSS to your theme:
table.horizontalMenu { text-transform: uppercase; }
To add this, go to Manage > Admin Control Panel > Themes on your site. Open/edit the theme you are using on your site and then expand the Custom CSS/HEAD Code section to reveal the Custom CSS field. Then paste the code above into that field.
If you are using a stock theme, you will need to create a new theme based on that stock theme and then edit it by applying the code above. Then be sure to set that new theme as your new master theme for your site.