There are some problems we all seem to "find;" this section documents a few of them. [Remember to always search before posting on the forums.]
Tables messed up, images not showing, other "strange" problems with HTML.
Drupal defaults to filtered HTML; that is, only certain tags are allowed. Further, that input format also breaks long lines of text. The fix is easy: switch to the "Full HTML" input format. You can make that the default for administrators (like yourself). Note that you may still want the URL Filter and Code Filter modules turned on for this input format; they are not defaults.
Help, I turned on Site Maintenance, now I can't login!
About three out of four Drupal users have done this to themselves. You can still log in with
http://www.example.com/?q=user. Note that "user" is exactly that - do not put your username there.
I don't want anonymous users to see "Create content."
"Create content" is actually a child menu item of "Content" which is usually disabled. Go to your menus administration screen and enable the "Content" parent above "Create Content." Then you will see a "Reset" link appear. Click on that. Once again, "Content" will be disabled, but "Create content" will not be shown to anyone who does not have the access permissions to do so (especially anonymous users).
I enabled the Contact module, but if I click on it as an anonymous user, I get the Access denied page.
You've missed a step. Go to
Administration >> User management >> Access control (or
Administration >> User management >> Permissions in Drupal 6,
People >> Permissions in Drupal 7) and scroll down to the contact module section. There you need to click on the check box for "access site-wide contact form" for "anonymous users" (and, presumably, "authenticated users").
Also read the handbook section Troubleshooting FAQ