Pharmacology requires at least basic knowledge in the following fields:
Biochemistry
Chemistry (particularly organic)
I would recommend starting with a generic 2nd year college level Biochemistry textbook, and have a similar 2nd year O-chem textbook handy (not necessarily a requirement, but it makes understanding things easier)
at the end of the day, pharmacology is like a cross between O-chem, biochem, and biology (particularly neuroscience).... but at your level the type of understanding to "start" understanding things comes from basic Biochem.
learn about the physics and chemistry of protein-ligand interactions, some of the physical chemistry relevant to this sort of topic, drug delivery issues such as integral membrane proteins that are involved in the detection and transmission of exogenous ligands, transport mechanisms inside the cell, etc.
these are all topics of consideration in pharmacology, because the field studies all aspects of "Drugs" from the moment they enter your body to the moment they are excreted from your body, as well as the natural compounds that these drugs mimic, and the ways that drugs and their natural equivalents are created (biologically) and used.
its a highly interdisciplinary field.
but its at least 30% chemistry, 50% biochemistry, and 20% biology/neuroscience.