Indoor rowing is a great aerobic exercise that can add variety to your training routine while engaging more than just the legs.
First learn to setup the equipment for proper use.
Good video instruction. I recommend you watch is a couple of times before jumping into a machine.
Now your ready to row, learn how to do it.
I started including the rowing machine while reading the first Tess Monaghan mystery, Baltimore Blues.
Phase 1–Drive (strong and quick)
Phase 2–Recovery (smooth and relaxed)
So after you've done a few minutes you will want to get some coaching about good form (what not to do). I found this video helpful and clear plus there are driller to help you improve.
For the experienced, none beginner, rower you'll want to do some sprints. This video from the same folks above lays it all out for you. I like to do a 10 pull sprint every 2 minutes but you should research this because I don't know what I'm done :)