How can I loop through a Channel entries asset folder?
I'm not sure about displaying "the first image from the asset folder" but, you can display the first asset as it's situated in the entry itself. Would that work? Maybe something like this?

{% set assetFirst = entry.myAssetsField.first() %}

{% if assetFirst %}
    {# or use getImg() (or either alias img/url): https://craftcms.com/docs/templating/assetfilemodel #}
    &lt;img src="{{ assetFirst.getUrl({ height: 400 }) }}" alt="image.title"/&gt;
{% endif %}

{% set assets = entry.assetsFieldHandle %}

{% if assets | length %}

    {% for asset in assets %}
        {# https://craftcms.com/docs/templating/assetfilemodel#simple-output #}
        &lt;a href="{{ file.url }}"&gt;{{ file }}&lt;/a&gt;
    {% endfor %}

{% endif %}