Kali Linux on Raspberry Pi and EW-7811UN WIFI

Good afternoon everybody.

I am a completely greenhorn on Linux *shame on me*
I was more or less successful in installing Kali on my Raspberry Pi. My goal is to set up the iSniff GPS Script. Now I stuck on my WiFi Dongle. I wasn't able to put my EW-7811Un into monitor mode. Did anybody successfully enabled monitor mode on this WiFi stick? I have Internet access over this stick so it seems to work generally.

Another problem I have is my graphical desktop. When i start the desktop with start x a black screen and mouse cursor appears. I figured out how to open a command prompt. After entering start x again in this box, the desktop appears. Any Idea about this? Is there a way to reinstall the desktop environment or something?