Search result on blank search query When you click in search form and press 'Enter' or 'Submit' button, it returns all entries, that exist in database.

How could I display some message rather than all entries?
You could use a conditional to check if your "search term" parameter has a value. 

&lt;h1&gt;Search Results&lt;/h1&gt;

{% set query = craft.request.getParam('q') %}

{% if query|length  %}

    {% set entries = craft.entries.search(query).order('score') %}

    {% if entries|length %}
        &lt;p&gt;{{ entries|length }} results:&lt;/p&gt;

        &lt;ul&gt;
            {% for entry in entries %}
                &lt;li&gt;&lt;a href="{{ entry.url }}"&gt;{{ entry.title }}&lt;/a&gt;&lt;/li&gt;
            {% endfor %}
        &lt;/ul&gt;
    {% else %}
        &lt;p&gt;Your search for “{{ query }}” didn’t return any results.&lt;/p&gt;
    {% endif %}


{% else %}
    &lt;p&gt;You forgot to enter a search term.&lt;/p&gt;
{% endif %}


(This code is modified from the examples in the Craft docs for "Search Form")