If you are new to Glocks, check out these sources;
www.glockstore.com www.glockworx.com www.glockbooks.com www.customizeyourglock.com www.glock.com .
There are many add-ons & replacement parts you can buy but many advise to keep your pistol factory stock.
I would agree with that in general but Id consider a Glock plug(to keep dirt, lint, grit out of the mag well), a good set of night sights like the Trijicon HD series or XS Big Dot & maybe a Hogue grip sleeve.
Clyde