Unless you stay under the radar, if you register as self employed in Greece you must register with EFKA (old OAEE Insurance) and pay minimum 185 euros per month insurance (based on Greece's minimum wage, so with up to 650 euros per month income. If income is above this the insurance increases by percentage). There used to be a get-out clause for this insurance if your work place was in a village, outside town boundaries, but I think this has ended.
VAT at 24% is chargeable and payable on services. Some of this can be reclaimed on your expenses relating to your work.
Income tax is 26% on the first euros of profit after insurance and allowable expenses such as office rent, phones, materials relating to your work etc.. with no personal tax allowance as you are seen as a 'business'.
You'll also probably have to pay an accountant to do your quarterly VAT returns and yearly tax return. I don't know any Greeks who don't use an accoutant for VAT returns.
Any Greek accountant can set it all up for you.