Good, basic introduction to some of the major themes.Biology LibreTexts - Receptors and Ligands
Biology LibreTexts - Types of Receptors
The Medical Biochemistry Page - Signal Transduction
Excellent, comprehensive (somewhat advanced) discussion of a very important topic for the MCAT. Don't let the advanced material intimidate you. Aim for comfortable comprehension, not knowing everything beforehand, not trying to memorize everything.