To start and continue an analysis, it is not necessary to obtain any knowledge about the theory. Even for the analyst, for whom of course the theory is necessary, his personal analysis remains the most important aspect of his training.
A theoretical study can never replace the experience of an analysis. For those who develop an interest in the analytical theory as such, these two works of Freud can be considered as excellent introductions: “The interpretation of dreams” and “Psychopathology of everyday life”. These works brilliantly elucidate that the unconscious is structured as a language.