The monbthly card called Abono Trasnportes can be bought at any 'Estanco' stall and you'll need to fill the personal data in the card and your picture or color photocopy of a picture and 1,20€ (probably a bit more on 2004). If you are under 22?? and ask for Abono Joven which is cheaper than normal you'll need to show the documents that confirms your age, DNI, or passport (may be a photocopy too). You'll need to indicate your area for the card, A, B1, B2....
They'll give you the card at the moment and you'll only need to buy the monthly ticket.
Make some calculations to see if you really need the card or is finally cheaper buying 10 travel tickets.
It's rare to find 'revisores' but i have found them a couple of times in the normal metro network and a few more in the places where you change from A zonce to B1 in the line 10.
And if you are thinking about entering without ticket, there is normally some guards at stations and in some places you'll need the ticket to get out the metro network.