you have to figure out which kind of lattice it is and what kind of packing the crystal has. For example there is body centred packing, hexagonal closest packing, cubic closest packing, etc. Each type of packing has a unit cell which has a certain number of atoms or particles. If you can find out the type of packing, then you will be able to figure out the number of atoms in a unit cell and then you will be able to figure out the coordination number.
Good luck.