I Stumbled across something today that I felt was worth sharing with the forgers in the bunch, both beginners and veterans. Tinkercad is a web app for Google Chrome, and its original purpose was to allow someone to design a product for 3D printing. But it can have other, more-applicable-to-Halo-Forgers, uses. It can act as an relatively quick way and easy to plan out sections of a map before you start building. You can brainstorm/flesh out that base idea you've had bouncing around in your head for a while, or plot out spacing and scaling between structures.
Those familiar with Google Sketchup will feel right at home. You may not have full feature parity with the rich Sketchup, but you have enough tools, in an easy to use, light interface, to get your ideas on 'paper' before you set off into Forge. Watch the video below for an introduction to the app. It'll teach you pretty much all you need to know in order to jump right in and give it a go.