Thanks

XNA Framework is a graphics API more than a game engine so you will need to borrow or write your own code to do terrains and editors. Riemers has a good tutorial http://www.riemers.net/eng/Tutorials/XNA/Csharp/series1.php. There are of course many other tutorials which can easily be found by typing XNA and TERRAIN into your favourite search engine....http://www.google.com/search?&q=xna+terrain

Or you could use one of the existing engines see http://forums.xna.com/forums/t/12882.aspx 

how would i use one of the existing engines for a creating a game?  Just use them like they normally would be used or do i have to link them with XNA somehow?

Depends on the engine. Some have GUIs that enable you to create 'worlds' and then you add code for the gameplay. Others you need to create everything through code, but at a higher level than XNA Framework on its own.

You will need XNA installed for them all to work.

Given the level of questions you are asking I would suggest you start off with some 2d simple games.

thanks