As far as "stuff" goes
"NEED"
1) ammunition
2) holster (Solely Canadian)
3) sights if you've got the plastic factory ones, since they can wear when drawing from the holster. In the short run the factory 'biscuit in the basket' sights can be greatly improved by blacking out the basket (white outline on the rear sight - it'll make you shoot better because the basket is just a distraction from correct focus)
"CONSIDER"
4) reloading press, bullets, brass, powder, primers - just in case you find ammo becoming 'import restricted' but also because you'll shoot more for the same money
5) An Uplula - thumbs are people too.
Other Important Stuff:
"TRAINING"
learn how to properly run the gun, from someone who knows how to run the gun. Training is an absolute hoot, you'll meet great people and develop a wider and deeper understanding of shooting. To me training separates the shooter from the serious shooter.
"LONG TERM"
You might want to get a couple of recoil springs - although Glocks are stupid reliable - some mag springs and a spare striker assembly for each gun. Be aware that you probably won't need these parts until you're north of 25,000 rounds. The mag springs will go first (virtually no one talks about these unless they shoot a lot, but they make or break reliability) then the recoil springs. Somewhere beyond 50K your striker may go south.