Regarding mobile telephony, Greece uses the same GSM system as most EU countries, Asia and Australia. You should have in mind that USA and Japan use a cellular phone system that is incompatible with the European.
You go to any Mobile service provider store (Cosmote, Nova Greece, Q-Telecom, Vodafone Greece) with your passport and ask for a SIM Card for your cellular phone, as well as a confirmation that this number belongs to you.
Alternatively you can make an online request at vodafone, what's up (cosmote), free to go (nova greece) or q-telecom and you will receive your card by post.
In addition to a residential address, you will need a Greek phone (mobile) simply to confirm the card's shipment.
The cost starts from 10 euros and the SIM card is being sent to you by postal service.
