Well... rowing machines are a great way to train your legs and to build up muscle.

Like eweaver15 said, getting started in rowing indoors is very easy - you just need to find an erg, or ergometer (aka rowing machine) to get started. Most people like to use it in gyms, because it is great for keeping a fit bum and burning your legs.
@iarvee: Sure, you can just start rowing, there is no need to consult a doctor. It's great that you have started for one week already. Do you swim on water or use the machines in gyms? I can give you tips for both on how to get the most out of your training.
@tbkwritiers: You are soooo lucky!!! I have to wake up a 4 in the morning just to get to the nearest river and 5 to row!!! If I was you I would row every single day and get to use the river!!! Try renting a boat or something and try it out, and if you like it then join a rowing club near you. I can guarantee that you will enjoy it.
To me, rowing is my passion.

And I enjoy every single second of it.