CAN protocol : How to start with CAN interface with Microcontroller
First of all, CAN is the hardware layers and CANopen is a protocol running on top of that. You will need to know how both CAN and CANopen work in detail.

Then pick a MCU which has a built-in CAN controller. This will save you lots of time and costs: avoid using external CAN controllers.

Then you have to figure out what kind of CANopen implementation the device uses. CANopen has a number of sub-standards for various applications, known as "device profiles". The device could be using one of the standardized device profiles or it could be using a custom one.

Depending on your requirements and what else is there on the bus, you might be able to get away with sending/receiving the known, hard-coded messages needed. If you want your MCU to act as a full-blown, standard-compliant CANopen device, then that's no trivial task. The sane solution then is to get yourself a pre-made CANopen protocol stack, either a commercial one or perhaps the free CANFestival.

Note that even if you find a stack with a port to your specific MCU, implementing it to get along with the rest of your application is usually not a trivial task. If you know nothing of CAN at all, and this is a one man show, prepare for a long project that might take you at least one year, probably longer than that.