Can you explain what the site is used for?
Hi, thats another thing I need to know. NicksGuide can be used for anything, I thought I did use it for reviews on consumer products. Good products, Bad products, product reviews, etc.
Any suggestions on this would be appreciated.
Do we set the new user as AUTHOR or EDITOR?
That depends on what you want users to be able to do. Read to documentation relating to users and decide http://codex.wordpress.org/User_Levels
Apart from this, how do you put different buttons on top (Home, About us, Marketplace, etc)?
The best person to ask is the theme author. Most of them are happy to help.