New to graphic design for OS X/iOS GUI - Ned some help!

1. Do I need to make vector images? If so what needs to be vectors (backgrounds)? Should I use Illustrator or Photoshop or both?

All major OSes uses raster images for GUI, it's however easier to build the graphics in vector to be able to scale the images for various purposes. I recommend using Illustrator and Photoshop (if you want to add a little zazz to the images, but really Illustrator should be good enough)

2. How do these graphics get implemented into the app using Xcode. Without going into too much detail would it be kind of comparable to HTML?

There is a user face builder built-in, the images you import into an assets foldr for images.

3. Do you implement all the graphics from images or can you use Xcode to do a lot of it (like CSS for web sites)?

not CSS, open a sample project and check how they are doing it. If you plan on using something like Phonegap there will be CSS in there, since it's 99% a HTML website.

4. What resolution & colour mode should I use in photoshop for GUI?

Depends on your target, iphones goes from 124 ppi to 324 (?) ppi, Apple Screens is around 124 ppi as far as I remember, but using a non-apple screen could bring you down to the norm of 72-92 ppi, check your target and focus on making it look good on the lowest quality first, then scale up using switches.

5. Whats the best method for developing retina & non-retiina graphics? Should I do everything in retina (@2X) as vectors then once complete resize everything down by 50% for non-retina?

I would recommend going the other way, make it low-quality first, then scale up, and when scaled you can start adding details...rather than scaling down having to remove details.