My mom struggles with seasickness and almost swore off cruising after a nasty experience for her. Overall, midship and lower decks are the best staterooms. Get one with at least a window (or a verandah), as looking out to the horizon can help your body 'adjust.'
What works for my mom: she started taking Bonine the night before our cruise. Last year she took it two hours before we sailed away, as the instructions said, and it did help with the seasickness but it made her terribly drowsy (like, falling asleep at Palo dinner drowsy). So she started the night before, let the drowsiness put her to sleep at a normal time, and it still worked. She also used scopolamine patches behind her ear starting the night before and changed them every few days per the directions.
We were deck 11 forward on this last cruise, so she was nervous that she'd feel more motion. She had one day where she felt a little dizzy (it was definitely rockier that day) but other than that, she was fine!