In the Dashboard go to Posts → Add New
On this page, you will see the WordPress posts editor. The most important parts of this page are:
Post Title - enter the title of your post in this field. On your theme, it will be shown above your content.
Post Content - you can use the WordPress WYSIWYG editor to add the actual content of your post. Note that it has two tabs - Visual (use the editor to format your text) and Text (add your HTML code directly).