I really like the Glocks stock. I have, however, inserted a Pearce Grip Plug into my G26. After having it strapped to my ankle a few days, I noticed it was accumulating quite a bit of "sock lint". Just a precaution though. I'm sure it would take a lot more than a little fuzz to gum up the works.
A big part of the decision on what accessory to get is going to depend upon how you intend to use the Glock.
If you mostly plan to carry for self defense, you need a good holster.
If you plan to spend a lot of time at the range, then you need a good range bag.
If you are having trouble with a specific part of the gun, you may need to go with an aftermarket part for that piece. Maybe you need an extended mag release if you're having trouble with that. Maybe you need better sights if you're having trouble finding your sight picture, etc. etc.
Naturally, some of the answers above, though meant partly as jokes, are also very accurate. You need ammo and LOTS of it!!! The best way to get to know your new gun is to run several hundred rounds through it. This will give you the feel for the gun, make sure it is operating perfectly, and will give you a little better idea of what other modifications might be needed to make it work perfectly for you!