I want to book a flight from Bucharest to Athens (arrives at 20:55) with Ryanair and to catch the flight to Thessaloniki at 22:05 with the same Ryanair. Do I have enough time to change the airplanes ? First time in Athens, I don't know at all the airport
First thing first, Ryanair is a point-to-point airline. This means that, as per their terms and conditions, they don't operate connecting flights. Moreover they can't and won't facilitate passenger and/or luggage transfers to connecting flights, regardless whether they are operated by Ryanair or not. Quoting from the linked T&C:
Article 17 - Point-to-point airline
We are a ‘point-to-point' airline. We therefore do not offer, and cannot facilitate, the transfer of passengers or their baggage to other flights, whether operated by ourselves or by other carriers.
What this means to you is that, after landing at ATH, you'll have to get your luggage from the baggage return area. From there you'll have to go to departures and re-check in the luggage in your connecting flight. Note that this is something you'll have to do both on the way to Thessaloniki and back.
You have 1h10m between the two flights. Now, the minimum connection time for ATH seems to be 45 minutes. However, this applies only to flights which are booked with a through ticket, according to the ATH airport website. It is hard to tell with enough certitude if 1h10m are enough to disembark, pick up luggage, drop the luggage off at the Ryanair counter, go through airport security and board your second flight.
One valid metric you can use to evaluate this is the fact that Ryanair baggage drop counters close strictly 40 minutes before the scheduled departure time. This reduces your connecting time from 1h10m to 30m. The chance of you making the connection will depend on a bunch of factors whose outcome is as unpredictable as it is unlikely to be in your favour. You'll need the incoming flight not to be significantly delayed, and that disembarking and baggage handling procedures occur swiftly. Frankly I wouldn't bet on it. Rather I'd arrange for a longer connection time.
As a general piece of advice, if you see that you are running short with time, don't hesitate to jump the baggage drop-off queue explaining your situation. This might be hard to do with Ryanair, but I'd try it nonetheless.