To add content to your site, you'd click on the link in your Navigation Menu that says "add content." This will bring up an overlay page that gives you all the content types available for creation on your site.
This listing will depend on your site.
Just click on the content type's name and the page will come up for that particular content type.
Depending on the content type you choose, there may be two or more fields to fill out on the page. Typically you will have at least a title and body. But other content types could have several more fields available. For instance, an event would have date and time fields and potentially ones for type of event, location, contact, and URL for more information.
Fill out the information on the page and then scroll down. You may see additional options that you can expand to change things such as the URL for the page (so you could have www.example.com/about_us as the address instead of www.example.com/node/24).
Once you're done with the page, scroll down and hit the "Save" button. Your information is not saved until you do that.