You could use the query:

SELECT threadid, dateline, title FROM thread WHERE visible=1 AND open=1 AND dateline!=4294967295 ORDER BY dateline DESC LIMIT 5;

But you would probably want to do some permissions checking for each.

Here is something to get you started:

Visit http://www.php.net. Read through the manual (you can skip the parts for most of the modules, just be sure to read the MySQL one). If you have done any programming before it won't be bad. Then go to http://www.mysql.com and read the documentation there. Then just go look at the /forum/index.php and just see if you can figure out how it displays the forums. Then check out /forum/forumdisplay.php and see how it displays the threads. Some other files to look at would include /forum/global.php, /forum/admin/global.php and /forum/admin/functions.php. Then you should have a pretty good idea of how to accomplish this, its not that difficult.