Hi there, welcome to PixelDam
As far as pixeling techniques, you should really go through this FAQ
and follow the links to the tutorial websites.
Some tips from me:
1. work out a rough sketch as a basic idea. See if everything fits, etc.
2. collect a bunch of good pictures of your scene as a reference for details, colors, objects
3. see if others have made a similar project and learn from them (mind you: not copying or stealing art) or even see if you can do better
4. make a screenshot of the block and make that your project 'background' (do not include in your submission though) in a sub layer if you work with photoshop
5. you can show us work-in-progress images to get early feedback