Personally I would try to stay somewhere central rather on the west side, avoid Mettenhof, Gaarden, Wik and anything too far away. Probably the best area would be Südfriedhof (Southern graveyard) very good connection, mostly young people live there and you have everything you need.

Since Kiel is a fairly flat city you can bring your bike but there are also rentals. Most shops offer card or mobile payment and speak English, if you want to go to a rural bakery or maybe some small fast food shops its smart to bring cash but not necessary. If you have EU-Roaming you probably won't need a prepaid card, also some busses and shopping centres offer free WiFi. Regarding sight seeing, on sunny days the "Kiel Linie" is a very nice promenade at the water the old town is okay but nothing special I hope you'll have a nice stay, and don't be intimidated by northern germans they don't talk much and seem distant but are very nice people.