Push-ups are among the most used strength exercises. But often, especially with beginners that may lack chest strength/movement awareness, the focused muscle becomes the serratus anterior, not the pectoral.
Arms that remain too extended or too much mobility of the shoulder-blade during the push-up are indicators that the form is not great.
Focusing on keeping a neutral spine and bending at the elbows can help to emphasize using the pectoral muscles.
Starting with the knees on the ground first can help to understand the movement better.
So bring the chest to the floor while flexing the elbows, and you should be all set for this exercise!