You definitely need to do this; looks like your material is copy-pasted from elsewhere on the web and it'd introducing a huge number of errors in your HTML.
The instructions may sound complex, but it's not difficult. Go to Dashboard->Settings->Writing and there's a tickbox that says "WordPress should correct invalidly nested XHTML automatically” and then scroll down and click “Save Changes." Just select it.
This means that in any post from now on that you make or edit, WP will fix all the code errors automatically. You don't need to worry about editing code.
Then you need to go into your last post and Edit it. Change anything at all: a comma to a period, or anything. Then hit Update. That will cause WP to fix all the code errors in that post.
Then do it for the post before that, and so on until at some point you'll have corrected the error which caused this. You don't need to Edit code, just edit the post.
Also, I hope you have the permission of those sites whose material you're using. Otherwise that's illegal.