You can create a Wordpress post from your Dashboard. hover over the Posts option on the left hand side and click Add New
Enter a title for your post and then add your post content, you have a tool bar that enables you to format the text, hover over any of these to see what they do, there is a Toolbar Toggle which enables you to open up more options
Click the Add Media button just above the text editor to add images or files, you can either upload your files, which will put them in the media library of your site, or you can input them from the media library if you have previously uploaded it.
Once you have created your post, click Publish to publish it!
That's it, your first post is done!