As self employed/ a freelancer in Greece you need to register as a small business. Have a look at this thread too - viewtopic.php?f=13&t=11152
You might try cash in hand work, difficult to say how much work you could expect, but it's illegal to work that way. You'd perhaps find odd jobs in a large expat area, or for a property maintence company (who look after holiday villas for absent owners).
I take it you are from the UK? One major thing to consider is when (if?) Britain leaves the EU next March, depending on the transition deal and how the Greek government applies that, you may no longer be entitled to live in Crete indefinitely, particularly as a newcommer, and setting up as legally self employed as a non-EU citizen could be nigh-on impossible. See half way down this page 'Rules for non-EU business owners' http://livingingreece.gr/2007/10/26/how ... in-greece/