Congratulations on the new lens, HIMS!
Shooting ultra-wide is really tricky in the beginning. I've owned my Sigma 10-20mm for a year now, and I still find it hard to compose the shots. You often tend to go much wider than you need, just because "you can" - so I guess my best tip is to not be afraid of shooting at the "less wide end" (sorry, my English is so bad) of the lens - 17-18mm is still pretty wide, even on a non-full frame camera.
I was aware of the problems with the Sigma lenses (some tend to be blurred in the right part of the frame at certain settings) when I bought mine, so I tried it for a week and made sure it was okay.
Here are some of my ultra-wide shots (that I'm pretty happy with):
Good luck!