I made an account, registered for free, and I keep checking Minecraft but even though I've been waiting all day I'm still a guest. Do I have to do something special to become a member or is it just because nobody's approved me yet?
Please read the top of the application page. It says you will receive a notification on our website when your app is accepted/rejected.
Then if you are not whitelisted in game and you got a acceptance notification you can contact a mod/admin. Most staff can promote you after checking your app to make sure you were accepted.