Reset the device without default configuration, create a bridge and add all interfaces as ports on the bridge. There you have your basic switch.
In addition:
- Add DHCP client to brdige, so the device can be managed on IP
- Add NTP client to make sure the device has the correct date/time
- Add VLAN’s if required (http://forum.mikrotik.com/t/using-routeros-to-vlan-your-network/126489/1)