Paste the following code somewhere on your index.php file using the theme editor:
<?php global $user_ID, $user_identity, $user_level ?>
<?php if ( !$user_ID ) : ?>
<a href="http://www.YourSite.com/wp-login.php">Log in</a> | <a href="http://www.YourSite.com/wp-login.php?action=register">Register</a>
<?php else : ?>
Welcome <strong><?php echo $user_identity ?></strong> |
<a href='<a href='<?php echo wp_logout_url(); ?>&redirect_to=/'>Logout</a>
<?php endif; ?>
The above code will show two text links:
Log In | Register
After logging in the page will show this message:
Welcome NAMEOFUSER | Log Out
As for a forum, you can use a forum plugin or something more advance like PHBB forum. If you don't know how to do this or the above, then just pay a freelancer to do it. [email moderated]