Metro tickets can be bought from an automatic ticket machine (multi-language available) or at any open ticket office. Additionally, you will find it in various places (newsagents, kiosks, etc.) throughout Athens.
You can use credit card or cash at all ticket offices and ticket machines.
You can use your valid metro and suburban ticket for 90 minutes as long as you touch in every time you change transportation without having to pay extra if you are on time. The remaining time of your ticket is marked on the card as well as on the machines and at the ticket offices.
More information about Athens transportation tickets can be found here.