From your screen i can tell two things,In the screen you are missing two things
Select any device you want from the list
Give memory below 700**
** on windows emulating ram more than 768M some times fail depend up on the system.
EDIT:
you can select any device from the list as you wish.If you are testing for a particular device select that one. For the first time you can select the nexus 4 that is listed first in the device and run your first app in that... happy coding