I will be starting my journey from Nishi-shinjukugochome station and I have a Suica card. Can it be used on the Joban Line train too? Thank you!
Yes, you can use Suica card on the Joban Line train.
I guess you are confused because Joban Line is a single train line but it does not directly take you to the last stop. You will need to transfer at least two trains to get to Mito because most of Joban line train goes only until Abiko, which is way before Mito.
Another point that is confusing is there is Tokyo Metro Chiyoda Line directly connected to Joban Line and takes you until Abiko.
So from Shinjuku-Sanchome station, you will need to transfer 2 times:
|[Tokyo Metro Marunouchi Line to Ikebukuro]
No.1 Trasfer to Tokyo Metro Chiyoda Line
|[Tokyo Metro Chiyoda Line (connected to Joban Line) to Abiko]
|*Changes to Joban Line after Ayase, no need to transfer
Abiko (the last stop)
No.2 Transfer to another Joban Line
|[JR Joban Line to Katsuta]
If train is too complicated, there are highway busses taking from Tokyo Station to Mito directly. Might want to consider that as well!
* Seems like Shinjuku - Mito highway bus is not operating now.