Which undergraduate classes are the most useful in pharmacy school. OChem, Microbiology, Calculus, Biochemm,...
A lot of what your curriculum will entail has to do with how drugs do what they do from the chemical and physiological standpoint.
I'd say some essential concepts.
Physiology- which organs detoxify. Filtration, hormone/mineral regulation, digestion, what produces what and where does it go.
Biochemistry- enzymes, regulation (allosteric, biofeedback mechanisms), causes/effects of chemical imbalances (biomolecule buildup/deficiency/syndromes: PKU, etc) prion based disease states
Anatomy: basic body part location, you don't need everything, but general information is very useful.
Organic Chemistry: Functional group naming, Le Chatlier's principle, pH = pKa + log [A-]/[HA], Schiff bases, amino acid chemistry.
GChem: Acid/Base chemistry, redox chemistry
Microbiology: (especially medical Micro) which bugs are major causes of disease, how antibiotics affect different bugs (cell wall, ribosomes, etc) Problem bugs (MRSA, VRSA, etc)
Psychology: Neurotransmitters, chemical imbalance based psych problems, neurotoxins, etc.
Not prereqs, but handy if you can get them
Medicinal Chemistry: Common ways body metabolizes drugs, structure function relationships, prodrugs.
Pharmacology: Drug names, actions, categories, antidotes, etc.
Pharmaceutical Chemistry: Same as medicinal plus some drug discovery/formulation techniques.
There are more than this, but these are some major ones. If you think, "Wait, that wasn't in this class, but it was in that one." It's because there are multiple cross-overs and some subjects are treated differently in different schools. Just pay attention when the above material is taught and you should have a good background for your first year.