Milan's metro system is typical of what you'd find in any large city. It's a bit dirty, rough…read morearound the edges, and littered with graffiti -- think of it as more New York City than Washington, D.C.
The metro system is easy to understand and well laid out with color-coded lines. Make sure you ask at your hotel before setting out which stops you want. There are plenty of useful stops all over the city and most are less than a few steps from the stations!
You'll encounter lots of interesting folks riding the rails, including impromptu musicians who'll give a concert of varying quality and then pass the hat. Of course, tourists should be on the lookout for pickpockets.
Keep your ticket because you need it to enter the turnstiles and also to exit! Once you insert it into the turnstile, it'll pop out the top. Take your ticket and enter. After your ride, you do the same thing at the exit gates.
You can buy your ticket in the machines located in each station but beware that they are in various states of disrepair. Try to have exact change because sometimes the machines run out of change and you're out of luck. They are very slow too, especially when giving back coins so be patient.
Each ride is a reasonable 1.50 Euros, but check out the special all-day pass that gets you unlimited rides for 24 hours. It's a bargain if you're planning on using the metro as your primary means of transportation.