The order of the bases in a gene is a code. The bases are read in groups of three, called a triplet. Each triplet codes for one amino acid. So 12 bases code for 4 amino acids.
This means the order of the bases in the gene sets the order of the amino acids in the protein. Different genes have different base sequences, so they make proteins with different amino acid orders.
The order of the amino acids decides how the chain folds up, so it decides the shape of the protein, and the shape decides what the protein can do. A change in the base sequence can change the amino acids, and so can give a protein with a different shape.