To delete an old post, go to the pull-down menu at your username in the upper right of your screen. Select 'My Content'. From the left side bar, select 'Only posts'. Find the post you wish to delete within the list of posts, then click on the ID number shown in the upper right corner of that post, Clicking the number takes you to your original post. At the bottom of your post, in the lower right corner, you should find a 'delete' option. Click it and confirm you wish to delete the post.
Let me know if this doesn't work!