How to add page numbers?

Hi Kamil, 
To add page numbers to the site. You can use the OB Page Numbers plugin.
Please find the plugin here: http://wordpress.org/plugins/ob-page-numbers/installation/
You can do as the following:
Step1: Activate the plugin ‘OB Page Numbers’ through the ‘Plugins’ menu in admin
Step 2: Open the index.php file then add the following code:

<?php if(function_exists('ob_page_numbers')) { ob_page_numbers(); } ?>

See the screenshot:
 
Step3: Go to ‘Options’ or ‘Settings’ >> ‘OB Page Numbers’ to change the options
Step4: Log in to Dashboard > Appearance > Editor > Open the style.css file then add the following code:
 

.paging-navigation {
   display: none;
}

#wp_page_numbers {
    margin-top: -40px;
}

Hope this helps!

That’s what you have written works, but there is one small problem (visible on the screenshot).

I guess I have to remove a line of code, but which exactly?

Did you mean to delete this part: http://prntscr.com/2654ra
If so, you can add the following code to the style.css file.

.paging-navigation {
   display: none !important;
}

#wp_page_numbers {
    margin-top: -40px !important;
}

Your link does not work, it may be easiest to paste some code that I have to delete.

Hi Kamil, 
Your screenshot is not clear to us what do you want to delete. We assume that you want to delete 1 one of the part like we highlight in this screenshot: http://prntscr.com/2654ra 
If so, please follow Dominic’s solution.

So I want to remove the first part of the file that is this:

I did everything I said Dominic, but the link that you serve (http://prntscr.com/2654ra) does not work and I can not delete the corresponding batch code:

Please send me your site for further checking.

Currently I’m building a site on localhost (WAMP), today ordered domain, tomorrow I buy a server and then I can send you the address.

Hi Kamil,

To resolve that issue, you can go to Dashboard > Appearance > Editor, open style sheet file style.css, and copy/paste the following code in:

.site-content .paging-navigation {
 display: none;
}
.site-content #wp_page_numbers {
 margin-top: -40px;
}
.site-content #wp_page_numbers ul {
 padding-top: 40px;
 border-top: 1px solid #e5e5e5;
}
.site-content #wp_page_numbers a,
.site-content #wp_page_numbers li.page_info {
 padding: 5px;
 padding-left: 10px;
 padding-right: 10px;
}
.site-content #wp_page_numbers li a:hover {
 border-color: #333;
}
.site-content #wp_page_numbers li.active_page a,
.site-content #wp_page_numbers li.active_page a:hover {
 border-color: #333;
 background: #333;
 color: #fff;
 text-decoration: none;
}

Hope that helps!