The monomer that makes up a polymer of protein is the amino acids. The amino acids are linked together to for a polypeptide chain, which can fold into the different proteins structures.
The amino acids are molecules that have one carboxyl end and one amino end, this characteristic make possible that each amino acid can link with other by the bonding between the opposite ends by peptide bonds. There are about 20 known amino acids that combine to make a protein by those chemical links.
Protein have linear, 2D and 3D structure that are made by the characteristics of each individual amino acid, often a functional protein is composed of more than one chain of amino acids, and sometimes requires small molecules (as some metals) to assist in its function and three dimensional structure.