If you are using select_tag without any other helper, then you can do it in html:
select_tag "whatever", "<option>VISA</option><option selected=\"selected\">MasterCard</option>"
Or with options_for_select:
select_tag "whatever", options_for_select([ "VISA", "MasterCard" ], "MasterCard")
Or with options_from_collection_for_select:
select_tag [SELECT_FIELD_NAME], options_from_collection_for_select([YOUR_COLLECTION], [NAME_OF_ATTRIBUTE_TO_SEND], [NAME_OF_ATTRIBUTE_SEEN_BY_USER], [DEFAULT_VALUE])
Example:
select_tag "people", options_from_collection_for_select(@people, 'id', 'name', '1')
Examples are from select_tag doc, options_for_select doc and from options_from_collection_for_select doc.