Start with general chemistry. Before moving on to organic chemistry, you should familiarize yourself with the basics about the structure of atoms and molecules (e.g. ideas about atomic and molecular orbitals, the geometry of molecules, lone pairs, polarity, resonance), basic thermodynamics (equilibrium, heats of reaction, entropy, free energy), basic chemical kinetics, and acid/base reactions (the ideas behind acid/base reactions are very useful for understanding many reactions in organic chemistry).