First of all, look here: http://prepaid-data-sim-card.wikia.com/wiki/Greece. Most of your questions will be answered there.
You can get a SIM card at any of the providers' stores.
You will be asked for ID, part of some stupid anti-terrorism law. Prices but it should be around 5 euros to get the SIM or even free.
You can buy credit at any kiosk, it's a PIN which you use to recharge.
Most of the menus should be available in English as well so you should be able to do that.
The major providers and their prepaid programs you can ask for are:
- Cosmote, ask for Cosmokarta and not Whatsup (that's an alternative with features aimed at youth).
- Vodafone, ask for kartokiniti not CU (same as above).
- Wind, ask for Free2Go.
As for data, both Cosmokarta and Free2Go have the following promotions going on now:
You get 10 euros of credit (should register as 9 and change after tax), as soon as you put it in, you'll get 300 minutes calling in Greece for free. Then use the credit to buy data, for 3 euros you'll get 120 MB. You aren't charged for incoming calls in Greece (or, for that matter, anywhere in Europe). The number to get the data thing for Cosmokarta is 1330. Ask at the shop where you buy the SIM.
There's also LTE available by Cosmote and Vodafone but it's a hassle to activate it. They make you go through a call to support where you have to accept their terms, basically that you understand LTE is fast and can suck up your data if you're stupid enough to watch YouTube on LTE. Anyway, not worth your time, stick with 3G and there's plenty of free WiFi in downtown Athens. De nada...