Click on Content | Article Manager.

The default view of the article manager is the most recent 20 articles that have been posted on the site. You can go to the bottom of the screen to change the number of articles listed. Or better, you could display only the articles you are currently concerned with by filtering the display. At the top of the list, you could filter by section, category, author and state.

Here's some detailed information about the Article Manager from the Joomla Help Site.

Continuing on with editing your article: find the article you'd like to edit, and click on its title. This will bring you to an editor screen. From the editor screen you can type in text, format text, and paste text. Take care when pasting text into a Joomla article. You can import photos from your media manager (link).

Most articles are easily editable, consisting of just text and pictures. But sometimes this is not the case. Be aware that some pre-existing articles could contain custom code developed for your site by your web developer. For instance, an article that corresponds to a shopping cart item might have tie-ins to paypal. Or a page that displays information from a database could have custom php code. Or your site may have had special requirements in how a particular article is displayed, so the article could be coded with html tables rather than just plain text. Take care not to overwrite custom coded articles.

Managing Content
Adding images, adding content, adding new articles

Managing Articles
New articles, images

Configuring Article Options
Inserting links, tables, display options

Next: Editing an article from the front end