Hi Heather,
I know there's an other route as well involving at least 2 transfers to Kiba. Depending on the amount of luggage it may be better to take take route, at least for the first time so you can get settled in. After that you could consider the walk to Etchujima, save some money and see some of down town Tokyo at the same time (or just walk the first time if you are lightly packed).
A Suica card (or Passmo) is an prepaid travel card (IC card), which you can use to travel around Tokyo and is valid on most kinds of transport, additionally you can use it to pay in many stores. An IC card is very handy so I would recommend getting one for sure.
As for transport in general in Tokyo, there's a lot of different companies which you can use to travel around the metropolis. There's the JR than there's the Tokyo Metro, the Toei Ōedo lines, the Yurikamome a driver less, elevated train and the Tokyo Monorail (this last one is covered by the JR Pass), just to name a few. These are all different companies, altough the JR will be able to get you to most important places, some may require you to use a different company too.
Hope this helps too!
Daniel