Recommendations:
The Core Book
The Runner's Toolkit (the cheat sheets alone are worth it!)
Seattle 2070
6th World Almanac
While some people don't like it, I would also recommend Attitude.(ESPECIALLY for the 1st two chapters)
As you are a new GM, I would advise against, initially, getting any of the books like Augmentation, Arsenal, Street Magic,
Unwired, or Runner's Companion. Those, while great books, are filled with advanced rules that really can bog things down.
That said, I do recommend getting them, and introducing them one at a time, as you get comfortable with them. Also,
the order I listed them? That is the order I would recommend introducing stuff from those books.