A beach is primarily formed by deposition. This process occurs when sediments, such as sand and gravel, are carried by the waves and currents and eventually deposited along the shoreline. Over time, these sediments accumulate, creating the beach we see.
While erosion can also play a role in shaping a beach, primarily by wearing away cliffs and rocks nearby, it is the deposition of these materials that builds the beach itself. Waves continually reshape the shoreline, eroding some areas while depositing materials in others. So, in summary, while both processes are at work, deposition is the key factor in beach formation.