Help design flaw

Hi guys

I have some problems with my first wordpress theme and i was hoping someone here could help me out.

You can find the website here https://www.m3kd.com/ufofiles

As you can see the blog posts are everywhere instead of just inside the maincontent div.

This is how it looks with just 1 blog post:

https://www.m3kd.com/ufright.jpg

And last this is how it is supposed to look when its done:

https://www.m3kd.com/uffinished.jpg

I hope someone out there can help me fix this!

Thanks ??

Here is the CSS

https://wordpress.pastebin.ca/984770

And here is the index.php

https://wordpress.pastebin.ca/984773