As there are no rowing videos to follow here try https://www.concept2.com/indoor-rowers/training even if your rower is not a C2. They have tips on posture and technique. As with all cardio styled machines outside of a staffed gym can be dangerous without proper training. Obtain as much knowledge as possible. If you are fortunate enough to be close to a rowing club, they will have machines there and you maybe able to find someone to give you tips on set up and technique. Who knows, this maybe your forte and they ask you to join them in a team!
We do not have rowing workouts on Grokker but i would recommend as a beginner you start slowly and build up over time. A great session is 20 minutes at a rate of 25-30 strokes per minute. It all depends on your fitness level and other workouts that you plan on doing.