Pick up the missions series, or one of the campaigns (Dawn of the Artifacts, Corporate Intrigue, etc.). Just like with D&D modules, these provide a good framework for a new DM to run a game. Also, grab the Runner's Toolkit. The two best things there are cheat sheets for common things (like spellcasting and matrix combat) and the Anatomy of a Shadowrun, which provides a side-by-side rundown of a run with a story on one side and game info on the other, including which rolls people are making.