implement wordpress in own site

I’m new to wordpress and i’m trying to integrate it in my existing site.

To do so i’m using the following code:

<?php
$args = array( 'numberposts' => 1 );
$lastposts = get_posts( $args );
foreach($lastposts as $post) : setup_postdata($post); ?><?php the_content(); ?><?php endforeach; ?>

This does the trick, but now i would like to only have posts of a particular categorie be shown. So i tried to add ‘category’ => 3 to the $args = array line. But if i do so, nothing shows up (and there are posts in this categorie). Any ideas why this is?

Also, in this case, i would like to only have the image of the post be shown, so no title, date (i already removed <?php the_title(); ?> and <?php the_date(); ?> so that wont show up), but the permalink and excerpt arte still there and don’t have their own <?php> tag in the code as seen above.. So can this be done?

The site im talking about https://www.moschalkx.nl/index1.php

Or does anyone know a good step by step information source for implementing wordpress in to your own site? Information in wordpress codex is a bit complex for me at this point.

Greets!