Log in at your Blogger account and you'll see your Dashboard, showing your list of blogs. Find a blue link called "New Post" as shown in the below screenshot, and click on this "New Post" link to write your first blog post.
Next, you'll find the Create New Post section under the "Posting" tab. Start by giving your post a title (optional), then enter the post itself.
When you're done, click the "Preview" link to make sure it's ready to go.
Once you're satisfied with your post, click the "Publish" button. This will publish your new post.