The G1 phase is the first step of interphase. While this may sound redundant, interphase altogether is also the first step of the cell cycle. The G1 phase the phase in which the cells grow in size and are starting to mature. Do not get this confused with the other phases, S phase, and the G2 phase. The S phase in which the cell begins to replicate its chromatin and the G2 phase is the phase in which the cell begins to start developing its organelles.
Sometimes G1 phase may go into G0 if the cell has not matured completely.