If you’re in great shape, do 100 correctly formed pushups as quickly as possible, taking breaks as needed.
If you struggle to do one good push up, try doing them on your knees, or stand at the base of the stairs and put your hands on the stairs. The key is to make it easy enough that you can do at least 10-20 repetitions.
Here are a few more ideas:
You can do pushups with your hands wider than shoulder width to get more of a shoulder and chest workout.
If you bring your hands together into a diamond shape near the center of your chest you will work more of the triceps and shoulders.
Working half pushups - either going only half way down and then back up, or starting from the floor and only pushing up half way and then going back down - will intensify your pushup workout.