To delete a specific post, just go to the Posts tab from your Dashboard, and hover over the post you'd like to delete. The Delete link should appear when you hover -- click that, and then confirm your deletion. To delete multiple posts at once, you can check the boxes next to all the posts you'd like to delete, and click the trash can icon.
When you delete a post, the post is only deleted from your blog. If you’ve posted your content in other places, such as a Google+ post (see example below), you will need to delete that content separately. Once you delete your blog post, any links to that blog post will result in a 404 error page.