J,

Hello and welcome to DotNetNuke! There are tons of resources here on the site at your disposal and a lot of people to support / help you. You should be able to find a ton of video tutorials here on the site on the video library page. 

With a DotNetNuke site you should be able to login from anywhere... location doesn't matter. You are probably actually getting logged into the site and it's just redirecting you to the home page. After you login does the text that was previously "login" now have your name on it?  If so then you've successfully logged in. 

In order to modify the menu structure and design you'd need to look into the skin file for the site which typically resides in the root > Portals > _default > Skins folder. In order to change which menu items (pages) show up in the site menu you'll need to set the properties for the pages in the "Page settings" section of the page.  Click around in there and look around. Should be pretty self explanatory.

Question: What version of DotNetNuke are you using? You can find this by logging into the site and clicking on HOST then going to Host Settings.  If you do not see the host option then you haven't been given host privileges.