Editing the posts display on the front page

Hello WordPressers,

I am completely new to designing my own WordPress theme. I am running the official website for a singer and since I am a huge fan of WordPress, I now want to install it on her website.

However, of course it should be a unique template. So for the last seven days or so I’ve been designing away. It’s not easy, but it is working so far ??

One question though:
I would like to change the display of posts on the front page. However, whenever I change something here, it does not only change the look of the posts on the front page but also on the single pages.
My template looks like this so far:

<?php if (have_posts()) : ?>
    <?php while (have_posts()) : ?>
      <?php the_post(); ?>
      <div class="post" id="post-<?php the_ID(); ?>">
        <h2 class="posttitle"><a class="posttitlelink" href="<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title(); ?>"><?php the_title(); ?></a></h2>
        <small><?php the_time('j. F Y') ?> <!-- von <?php the_author() ?> --></small>
        <div class="entry">
          <?php the_content('Den ganzen Beitrag lesen »'); ?>
        </div>
        <p class="postmetadata">
          Kategorie
          <?php the_category(', ') ?> |
          <?php edit_post_link('Bearbeiten', '', ' | '); ?>
          <?php comments_popup_link('0 Kommentare »', '1 Kommentar »', '% Kommentare »'); ?>
        </p>

Whenever I change something here, it does not only change the look of the front page. So does anyone have an idea?

I started out with a template set by TemplateR, which seems to be very basic, for example I do not even have a functions.php and do not really know what it is and if I need it ??
Sorry, but I’m still very new to everything.

Any help is greatly appreciated.