Start by doing wall pushups. Stand facing a wall and place your hands on the wall at shoulder height, slightly wider than shoulder-width apart. Lean in towards the wall and bend your elbows to lower your body towards the wall, then push back up to the starting position. Do 3 sets of 10-15 reps.
Next, try knee pushups. Get into a plank position with your hands on the ground, but with your knees resting on the ground instead of your toes. Bend your elbows to lower your body towards the ground, then push back up to the starting position. Do 3 sets of 10-15 reps.
Once you can comfortably do knee pushups, try doing standard pushups with your feet on the ground. Start in a plank position with your hands on the ground, shoulder-width apart. Lower your body down towards the ground by bending your elbows, then push back up to the starting position. Do 3 sets of 8-12 reps.
As you get stronger, try increasing the number of reps you do for each set, or try adding in additional sets. You can also try doing pushups on an unstable surface, such as a Bosu ball or a stability ball, to increase the challenge.
Remember to warm up before each workout with some light cardio and dynamic stretches, and cool down afterwards with some static stretches to help prevent injury. As with any new exercise routine, it's important to listen to your body and take breaks when needed.