First, I'm unable to flush the cache. When I try to load http://localhost/admin/?flush=1/, I get an error will the following information. Line 120 appears in red.

[Notice] Undefined variable: _SESSION
GET /admin/?flush=1/
. . . . .
119       // Load the session into the controller
120       $session = new Session($_SESSION);
121       $result = Director::handleRequest($req, $session);
122       $session->inst_save();

Also, I don't seem to be able to create a new page type. I followed the instructions saying:
"Create a new file HomePage.php in mysite/code. Copy the following code into it: "

When I try to change the type of page in the CMS though, HomePage still doesn't appear. What am I missing here?

That doesn't seem to change anything. I changed the file name, restarted Apache, but I still don't see a different page type. Not being able to know whether the cache is flushed is pretty annoying too--that seems like it should be a straightforward task, but I still get the same error.

I'd like to learn SilverStripe because a good friend of mine uses it for work. It's frustrating though, not being able to learn more because I can't create my own page type or flush the cache using the provided instructions. Am I missing something simple here?