I want my post to show all the words. So i change this value in post subtemplate from the_exerpt to the_content. But after I've done that my post is not showing all the content, but something like in the attached image. I think the answer to this question is in these links:

http://codex.wordpress.org/Customizing_the_Read_More
http://codex.wordpress.org/Template_Tags/the_content

But I am not a coder, and I have been trying to solve this problem on my own, but it seems like I need some advice from you guys. Could you give me some tips where and which code to edit ?

What version of WP and Montezuma?
What is the URL?
Have you tried disabling ALL plugins at the same time?

Ad.1
WP 3.9
Ad. 2
http://www.projektowaniebim.pl/
Ad.3
I tried it after reading your post. I turned off all of the plug-ins- it didn't help.

It looks like you are using the Post Teaser plugin. Deactivate that plugin and you should be able to see the entire post contents.

Please remember to add a link to your site whenever you have a question, it will make it easier to see what is going on.

Yes it was a plugin, the one which should help you customize your start page withe deleting Hello world and messed up my permaling structure.

The page address is
www.projektowaniebim.pl