Identified by a large M in a circle, metro stations are easily spotted. The network of lines covers central Paris, and every station has clear indications, so you are unlikely to need help. Lines are identified by their end station, only two have branches, which means you have to check destination on the front of the train.
Doors open easily : lift the handle or press the button, and they shut automatically. Connections are called correspondances, and an orange sign on the platform indicates directions to your connecting line. Blue signs marked sortie indicate the exists.
One ticket is valid for every uninterrupted journey. After braving the automatic barriers, keep the ticket on you until you end your journey in case of inspectors.
Parisian public transportation is one of the best systems in the world. Metro maps are available free from any metro station and from some hotels.

You can also see paris metro subway map.