MAC addresses are assigned by the IEEE registration authority (www.standards.ieee.org).
Usually a company buys a bunch of addresses to be used on its products or registers its OUI (Organization Unique Identifier) which is hereafter associated to the first 3 bytes of MAC address.
If you are using a development board with network connectivity, the supplier usually provides you one of its own MAC addresses to be used on it.
Every MAC address should be different from any other in the world. Anyway if you are only making tests, I mean you are not deploying your board to the field and use it on a private LAN, you can possibly use a random MAC address.