Your site’s normal font size is around 15 points = 1 em. If you do not format your text, it will already have a default style. You don't have to do anything to it. Just write your text and save. You can bold and italicize your text.
NOTE: Don't use h1 headers in your regular text content. This is usually reserved for page headers and are automatically generated by Joomla. The search engines look for those as your page title. And Web readers for handicapped also look for page headers as a place to start reading. If you have more than one it confuses the reader.
Remember that, regardless of your site’s font size, people are able to change the size to whatever they like within their browsers, so you don’t need to have very large fonts. If you make fonts too large on the site, it may make the layout fall out of whack since it pushes on the structure of the site. Think of your pages built out of blocks that have content within them. This is specially true in most modules. And in the Web, as well as in Texting and Emails, large text and Caps equals screaming! Not a nice way to talk to your clients.
Play with it and see what happens.
If you play with a bogus article (one that isn't linked from anywhere), you can save it as long as it isn’t associated with any category it will not display in the site. You can use it to experiment on. This is a good way to get accustomed to the editor without causing some serious harm to your content.
The largest page title at the top of your page is an H1 title and the style has already been predetermined. You cannot change the style. To make changes to the content in the title, you must do it in the menu for the page. To edit a Header, go to the main Joomla navigation and find Menus. Find your Main Menu and then find the menu item you want to edit. Click on the menu to open it. Then go to Page Display/Page Heading. Edit the title and save the menu.
Blog Page Titles
A Blog is a page made up of a list of articles belonging to one or more categories. This creates a listing. You click on an article on the list and it takes you to that article.
If the This is an example of a Blog page. On the left you have a list of posts or articles. On the right, you have a listo of latest posts.
Good Management Practices
It is a good idea to plan your content before you build it. Having a good outline of what you want to achieve is always going to have the best results. If you have several articles to add, think of what you are going to need: images, content, PDFs, etc.
Naming your articles exactly what they are is a good thing. Think of how you will find them later, or even better, if several people are editing the site, make it easier for others to find the articles. This is true unless the articles are being automatically displayed in pages in Blog styles.