It is all explained here:
http://www.kotsu.metro.tokyo.jp/english/images/pdf/bus_...1)Where to purchase the ticket?
In the bus.
2) From where the ride starts? Is it at Tokyo Stn.?
It operates between Tokyo Station and Ryogoku.
3) How good is JPY200 for?
One ride.
Let's say, if I get off the bus at Ueno Stn., do I have to pay another JPY200 when I get back for the remaining ride?
Yes, you have to pay for another ride. Or buy a day pass for 500 yen.