Problems with div's, margin and padding not working

Hello everyone, I hope u are OK.
I need your help or ideas for my CSS problem.

And I want to erase that margin 300px, and put sidebar under preview-post without it.

When I’m adding margin-bottom or padding-bottom to preview-post its still the same.

Any ideas? I’ve tried with postition: absolute or display: block, but still dont works..

<?php get_header(); ?>

<div id="container">
                <div class="preview-post">
                        <div class="preview-content">
                                <div class="entry-meta">
                                ....
                                </div>
                        </div>

                </div><!---. preview-post-->

        <div id="primary" class="sidebar-class">
        <?php get_sidebar() ?>
        </div>

        <div id="post-recent">
        ...

        </div><!-- .post -->

</div><!--- .containter -->
<?php get_footer() ?>

.sidebar-class{
        margin: 300px 0px 0px 0px;
        font-weight: 500;
        width: 226px;
        position:relative;
        float: left;
}

.single .preview{
        height:300px;
        margin:2px 4px 0 2px;
        width:226px
}

Everything looks like that:
http://oi46.tinypic.com/3324c2q.jpg.