I picked Oakland ARTCC because
1: It's where I live in real life.  
2:  They have ample amounts of air traffic going through it
3:  They seem to have enough instructors and mentors available and willing to teach so that your training will never get stuck.

I think that the last two are the most important points.  You might want to try looking at the forums of the ARTCC that you're interested in joining, as some of the quieter ARTCCs haven't had posts in months.

I signed up for the ATC test, then it automatically started one for me.  So I took it, but then when I clicked on "grade test", it asked me to select an ARTCC.  

How do I know what my grade was, and whether or not I passed?  

Or does the selection screen mean that I already did pass?