The first step in turning a rock into a sediment is Compaction
.
Explanation:
Lithification is a process of changing rock into sediments. There are two steps for a rock to lithify. These steps are as follows
The first step of lithification is compaction where sediments are erosed together by weight of the it. Thus, the upper layers of sediments causes compaction of lower layers.
The next process of lithification is cementation. In this fluids fill the space between the loose particles.
Hence, the first step for turning rock into sediments is compaction.