If you are trying to get your first pushup or perfect your form so you can do more pushups, start with a plank. The pushup is a moveable plank. The pushup starts in a good plank position with the abs and glutes braced and the shoulder blades retracted or pulled back.
This position should be maintained throughout the entire pushup movement. The plank position gives the body the core strength and stability to do a proper pushup.