I have been in Lisbon in September for one week. And let me tell 'ya I had a jolly good time.
First of all, if you visit Lisbon in the summer beware of the constant heat or the real nice weather. The time you step out of the airport you should have your sun glasses ready. If you go by bus to the city center take line 44 or 745. You pay 1.75 € for a single ride, whereas you pay 3.50€ in the aerobus. You can buy your ticket from the bus driver and don't be shy, most of Lisbon's people you'll get in touch with can speak English. A friend told me that is because they get all their TV-series only in English with Portuguese subtitles.
If you plan on using public transport you can either buy your ticket from the bus or tram driver, which has the disadvantage that you should always have some change at hand or you get yourself a 7 colinas card with the option zapping at a cost of 0.50€. Thus you can load for example 10€ on your card and get around Lisbon using tram, bus, metro or even train. You only have to validate your card at the entrance of each vehicle. You can get the card at the tourist information at the airport or at each ticket autmomat in metro entrances. For me charging the card once with 15€ was more than enough for my one-week stay. Although I challenged myself, walking up and down the city's steep hills. But hey, even the old Lisboetas can do it, so I can do it, too. And it makes for perfect muscles in ass and legs. Also necessary to know is that when you wait at a stop you have to indicate the driver you want to get on. So stretch out one arm or otherwise the driver will not hesitate and somewhat coolly drives by. And if you have trouble reading a timetable. The number in a circle means that a bus or whatever is coming by every "circled number" minutes (this is at least different from German timetables).
For people who have never been to Lisbon before I wholeheartedly recommend a Lisbon Walker Tour (clicky clicky). They have fantastic guides who know to give a lively tour and tell funny anecdotes. I joined the Lisbon Revelation Tour with Inès, a friendly and knowledgeable tour guide. She showed us around town about three hours including a ride on the Santa Justa Lift and a ride in the famous Number 28 tram. As a student I only had to pay 10€, whereas a normal adult ticket costs 15€. Tours start at Praca do Comercio nearly everyday (check the timetable at their site) at 10 o'clock. You can get free maps of the city at every tourist information. But you can't miss Praca do Comercio it is next to the river Tejo, in the heart of the city.
I plan on two more posts about Lisbon. One about the top five things I did in Lisbon like Europe's biggest aquarium and a post about trips out of town like swimming in the Atlantic Ocean.