The Lisbon metro is cheap and fast for getting from the city centre to some of the suburbs and vice versa. The stations are clean and the trains are efficient. There are only four lines which are colour coded (green, blue, yellow and red) so it's easy to see which line you want. Each one is clearly signposted. A single journey costs 70 cents but it is much cheaper to buy a book of 10 tickets for 6.50 euros.
Updates
Is this your business?
Look out for pickpockets - one trick is to hop on, stand in the doorway and 2-3 conspirators close in behind and go through your stuff as you try to get by before the doors close (when they make a fast exit and leave you minus your valuables).