The closest station where you can buy tickets is Evangelismos Metro Station. The alternative is Syntagma Square Metro Station if you in the downtown area. In the linked site there are sections that are in Greek (although you have switched to English). Most of the info that is not translated refers to long stay customers that want to issue monthly tickets.
The link will take you to a FreeNow taxi service page. They have an App. You don’t need to use their service for which they charge a small additional flat fee. My point here is to link to a page that hopefully will be promptly updated if the Government regulated taxi charges change. The other App you can reliably use is Uber. Note that in Greece Uber calls a regular taxi.
I used to prefer FreeNow but got upset with their pricing policy (unfair to taxi drivers) long ago and switched to Uber. I think, in the meantime, they have changed their policy (now it seems they charge the customer, maybe also the taxi driver). Download both Apps and use them interchangeably particularly in the suberbs. If you are travelling in the downtown area, you wont have a problem finding a taxi at any time of the day. You can also hail down a taxi. Very easy in the downtown area.
The benefit of using the Apps is that your ride is registered on the system. If you forget something in the car, you ride is traceable and your chances of finding it much higher (unless the next customer took it!).