How would I make a basic select list?
This is taken directly from Form API Reference. Familiarize yourself with it.

$form['selected'] = array(
       '#type' =&gt; 'select',
       '#title' =&gt; t('Selected'),
       '#options' =&gt; array(
         0 =&gt; t('No'),
         1 =&gt; t('Yes'),
       ),
       '#default_value' =&gt; $category['selected'],
       '#description' =&gt; t('Set this to &lt;em&gt;Yes&lt;/em&gt; if you would like this category to be selected by default.'),
   );


EDIT

To print the result from the selection, you could consider a few things:


Do not use a form at all. Just use theme_item_list wherein you create hyperlinks to pages with nodes or users.
Use the form, but persist the user's selection (node or user) to the database or SEESION. Then when the form is created you reload the persisted value from the database or SESSION (if not present use default) as the default value so it always reflects the user's last choice, and you create a markup element wherein you show either the nodes or users that were the result of the selection or the default. In the submit you only persist the value to the database or SESSION. In the form creation you look at the persisted or SESSION value (if present, else use default), and display it in the markup element.
Use the form, do not persist anything in the database, and in the submit you inspect the select list value, and depending on the value you redirect (using $form_state) to a page that will show either nodes or users.
Create a jump list: for example ARCHIVE: Example: How to Create a 'Jump Menu' from a List View