In Lisbon, Subway stations can be identified by the red M (which stand for Metropolitano, which is what we call our subway).
Here's a regular entrance:
At the entrance you're likely to find these two posters. The one on the left has the working hours of the station, which in this case is from 06:30 to 01:00. I can't assure you that all stations have the same schedule, but they're likely to have.
With some stations, some of the entrances close earlier than what is advertised. This means that if, for instance, it's 00:00 and you see the entrance closed, you should look around, because there should be a bunch of others (each station has about 4 to 7 entrances) that happen to be open.
Inside the station you will find the diagram of the subway network. There are four lines.
You can also find it online at http://www.metrolisboa.pt/diagrede_uk.htm
If you click the train station in that online diagram, you will see some information about it... (example: "Estação do Oriente")
You buy tickets in these machines:
Apart from Portuguese, these machines have a menu in English, French and Spanish.
You will usually want a "single ticket", which costs 0,70.
The ticket you want is likely to be the "1 zone" ticket. You will only need a "2 zones" ticket if you're going to the stations of Amadora-Este, Alfornelos or Odivelas, which I wouldn't expect you to, but you never know.
I should also let you know that most people in Lisbon who have to go to those stations prefer paying 30 cents less and walk the rest of the journey, which is not that far.
The machines accept Euro coinage and bills, and also some cards.
You will use your ticket to pass through these:
Do notice the following: at times, these passages are open and you can pass without using your ticket. I do not recommend doing that, as there are sometimes people checking whether you're doing everything properly.
In the subway, among other things, you will find maps of the city.
You will also find smaller maps of the city area the station you're in is.
Remember: the train runs on the left side of the rails, which means if you face the line, your train should arrive from your right. In some stations (not many), the train isn't long enough to cover the whole station, which means you should wait on the left side, in order not to have to run to catch the train.
Trains arrive every five minutes during most of the day, so there shouldn't be any need to be in a hurry.
The subway system is usually quite safe and there's security patrolling it.
That doesn't mean you shouldn't be careful, but it is safe :-)
In the early morning there's people distributing newspapers ("Metro" and "Destak") for free.
Most of the stations have some kind of art (paintings, sculpture, etc.), so keep an open eye for it ;-)
More information on the subway system in Lisbon can be found at http://www.metrolisboa.pt/index_uk.htm