Turn on the walled garden from your site advanced settings

I think it's label is "Restrict site content to logged in users" or something similar

I like that very much and up to a point it gives me a good restriction although it is markedly too much. I'd like to have a fronmt page more like the one for ELGG itself. How do you do that?

http://elgg.org/index.php

it gives you some general news and does restrict to none inscribed people or maintanins members to a certain access.

The source code for elgg community is opensource and is available at http://github.com/elgg

In that case you must write a theme (or use the code provided in the link above), in order to achieve appareance you want, combine Matt's answer and your theme and you should get what you want.

An easy way to start is to modify topbar menu, if your users are not geeky enough you can hide links and they won't access certain pages, but of course this is a temporal solution in the meantime you write your theme.

Test for a logged user using:bool elgg_is_logged_in()

Remove elements from topbar or site menu: elgg_unregister_menu_item("site", "thewire");

And add elements to topbar or site using:

elgg_register_menu_item('topbar', array( 'name' => 'profile', 'href' => $viewer->getURL(), 'text' => 'Editar mi perfil'));