Can't really comment on whether it's your fault or not, or whether the warranty applies. I'd say take it back and try. Only way to find out...
Now onto actually doing it yourself, if you were so inclined. Taking apart a hub is pretty easy and straight forward. The only specialty tool you need is a cone wrench, which is just a really slim wrench, to get into the cone-nuts. Knowing about the concept of a locknut is important here. There will be a 17mm nut locked to a 15mm cone nut on the shimano hub you have, so you'll need a normal 17mm wrench as well.
If you're working with a rear hub, you want to undo the nuts on the non-driveside and pull the axle out, leaving the driveside nut-cluster in place. For a front hub, you can remove either side. Be careful, as bearings will be exposed and can spill out. You can then inspect the axle to make sure it looks good. Also inspect the amount of grease in the bearing races, and you can inspect the races for pitting or wear also.
I've seen bearings split, dirt get in, axles break, and cone nuts get loose, among other things. Any of these issues could cause your current problem.