ok,
1. How can i add my own logo instead of the blog name in the header?
create a child theme, copy header.php into the child theme and edit it:
change this section:
<hgroup>
<h1 id="site-title"><span><a href="<?php echo esc_url( home_url( '/' ) ); ?>" title="<?php echo esc_attr( get_bloginfo( 'name', 'display' ) ); ?>" rel="home"><?php bloginfo( 'name' ); ?></a></span></h1>
to something lke this:
<hgroup><img id="site-logo-image" src="https://yoursite/wp-content/uploads/2011/07/logo.jpg" alt="" />
2. The headline of the articles is in the single post page bigger than in the blog page. How can i change this?
this is set in style.css:
.singular .entry-title {
color: #000;
font-size: 36px;
font-weight: bold;
line-height: 48px;
}
add something like this to style.css of your child theme:
.singular .entry-title {
font-size: 26px;
}