The first step in adding a page in Joomla is to understand how Joomla displays pages. To display page, Joomla uses what it calls articles. Articles can be assigned to an infinite number of pages.

To create new articles, you must first sign in to the backend of you website:
http://your_domain_name.com/administrator

To see your current articles, click "Article Manager" from the Content drop down menu.

 

To create a new article, click the "New" button. 

Then make the following changes:

Title: Some Title
Alias: (leave it blank)
Section: Uncategorized (This is used for organization of your articles. You can add sections from the content menu)
Category: Uncategorized (This is used for organization of your articles. You can add categories from the content menu)
Published: Yes 
Front page: No (You can display this article on the front page if you would like to.)

In the field below, you will see a WYSIWYG editor (What You See Is What You Get). This is where you will create your Article.  You can put text and images in here. You can use the tools to bold or underline words, create tables and more. 

Once you are done creating your article, click save.

Now that you have an article, you now need to know how to get to it from the front end. I actually have already discussed this in a previous blog and it can be read here:
http://lab11.com/website-design/blogs/4-twmeier/11-understanding-joomla-urls

I would also suggest reading another blog that explains how to create Search Engine Friendly URLs:
http://lab11.com/blogs/4-twmeier/13-creating-search-engine-friendly-urls