I know nothing about incredimail, but here is how you do it in Outlook Express.
Launch Outlook Express, click Create mail, and a new box will open. Go to format, (on the top), and select Rich Text (HTML). As soon as you start typing a message the box giving the choice of Fonts, their style and colour will open. The range of fonts available will depend upon those installed on you PC.
Remember that some recipients, particularly businesses, will not accept messages sent in HTML format.