First, read "Ender's Game" - it's the core book and establishes the setting.
Then I'd read "Ender's Shadow", which covers the same period of time from alternate points of view.
After that, it really depends on which plot you want to follow. There are two trilogies, and they go in very different directions.
The Speaker trilogy (Speaker for the Dead, Xenocide, Children of the Mind):
follows Ender after he leaves Earth, and takes place in the far future. It tends to focus on "meaning of life and universe" topics.
The Shadow books:
stay in the "present" on Earth, following Bean, Peter, Valentine, and the other supporting characters, and explores the aftermath of "what happens when a bunch of teenagers save the world". These books are a bit more political and realistic.
The short stories tend to be snapshots in time, and aren't really necessary until you've become well and truly hooked.
For instance, "Investment Counselor" introduces you to Ender's AI, but it works perfectly well as a flashback instead of reading that in it's chronological order. War of Gifts is similar, in that it fills in history without advancing the plot.