If you look at your solrconfig.xml file, you will see that the /select requestHandler is configured with the default search field as:
<lst name="defaults">
<str name="df">aField</str>
</lst>
Here, you can put the field that you want like for example:
<lst name="defaults">
<str name="df">name</str>
</lst>
You can see another topic about this subject and the Solr Documentation.
To build a searcheable field, in your schema.xml, you can create a new field:
<field name="searchable_field" type="text_general" indexed="true" stored="true" multiValued="true" />
After that, you can copy the fields that you want in your new field:
<copyField source="name" dest="searchable_field"/>
<copyField source="last_name" dest="searchable_field"/>
<copyField source="birthday" dest="searchable_field"/>
<copyField source="location" dest="searchable_field"/>
And to finish, you just have to put this new field in your solrconfig.xml:
<lst name="defaults">
<str name="df">searchable_field</str>
</lst>