Two plus one
Beijing currently has two subway lines and one recently inaugerated light railway. The subway is very useful, but you
are unlikely to use the light rail unless you are studying or living in the Wudaokou area in Haidian.
The subway
Two lines, one's a circle, one's a line. Any journey costs 3Y, and you can change lines at two intersection stations if you need to for no extra charge. There are sometimes queues for tickets, but they are usually orderly and fast-moving.
After buying 
your ticket from one blue-uniformed woman, you immediately hand it over to another one at the top of the stairs. Go down the
stairs and look at the signs on the wall of the subway that indicate which direction the trains are going, and get on the right one.
Like any large-city subway, it can get
very crowded. Stations are announced over the tannoy in Chinese and English, and if you can see out the window there will be
signs with the name of the station in characters and pinyin. When you get off, find the map of the immediate area on the platform -
this will show you the surroundings with all the exits from the subway station clearly marked, and it's useful to figure out
which one you need.
Coming out of the wrong exit can result in a twenty minute walk to find a pedestrian underpass to get over a motorway, or paying another 3Y to go back down to the platform and come back up out the correct exit. If you are arranging to meet someone at a subway station, its usually easiest to specify an exit - they are named alphabetically, or to meet on the platform (unless it's an interchange station with two platforms). If you are taking a bus when you leave the subway, look on the same map for the bus stops.