I am trying to create a simple frontend form for posting

To post from the front-end you can use wp_insert_post() function.

So its simply a matter of a form and processing it

the form:

 <!-- New Post Form -->
<div id="postbox">
<form id="new_post" name="new_post" method="post" action="">
<p><label for="title">Title</label><br />
<input type="text" id="title" value="" tabindex="1" size="20" name="title" />
</p>
<p><label for="description">Description</label><br />
<textarea id="description" tabindex="3" name="description" cols="50" rows="6"></textarea>
</p>
<p><?php wp_dropdown_categories( 'show_option_none=Category&tab_index=4&taxonomy=category' ); ?></p>
<p><label for="post_tags">Tags</label>
<input type="text" value="" tabindex="5" size="16" name="post_tags" id="post_tags" /></p>
<p align="right"><input type="submit" value="Publish" tabindex="6" id="submit" name="submit" /></p>
<input type="hidden" name="action" value="new_post" />
<?php wp_nonce_field( 'new-post' ); ?>
</form>
</div>
<!--// New Post Form -->

the processing:

if( 'POST' == $_SERVER['REQUEST_METHOD'] && !empty( $_POST['action'] ) &&  $_POST['action'] == "new_game_post") {

    // Do some minor form validation to make sure there is content
    if (isset ($_POST['title'])) {
        $title =  $_POST['title'];
    } else {
        echo 'Please enter a game  title';
    }
    if (isset ($_POST['description'])) {
        $description = $_POST['description'];
    } else {
        echo 'Please enter the content';
    }
    $tags = $_POST['post_tags'];

    // Add the content of the form to $post as an array
    $new_post = array(
        'post_title'    => $title,
        'post_content'  => $description,
        'tags_input'    => array($tags),
        'post_status'   => 'publish',           // Choose: publish, preview, future, draft, etc.
        'post_type' => $_POST['post_type']  // Use a custom post type if you want to
    );
    //save the new post and return its ID
    $pid = wp_insert_post($new_post); 


}

Bonus
Once you have the post id ($pid in the above example) then you can easily set terms and custom fields:

 //insert taxonomies like categories tags or custom 
    wp_set_post_terms($pid,(array)($_POST['cat']),'category',true);

//insert custom fields
update_post_meta($pid,'meta_key_name',$_POST['meta_value']);

hope this helps.