AMKA is the labor and national insurance number in Greece and is the number/card you need to be allowed to work and access welfare services here. Applying for an AMKA is free and can be done at any KEP (Citizen Services Centre) office, KEM (Center for Integration of Migrants) office, or IKA office. Be aware that all procedures are conducted in-person and in Greek, although some staff members may speak English.
The process to get an AMKA has now been made easier for people in Greece who have been granted asylum or who are applying for asylum. Now, you can apply to get an AMKA with just your white card or your residence permit. This is also possible for minors. You should be ready to provide an address if one is not listed on your document. This recent change in the law should have been clearly communicated to all the responsible offices, but just in case, you can print the directive document (available here: http://bit.ly/2CJ46nA) and bring it with you so staff members are aware of the new procedure.