Here's a pushup routine to get you started:
With a continuous running clock do 10 pushups every minute on the minute for 15 minutes. So start the clock and do 10 pushups, when the clock gets to the second minute do 10 pushups, when the clock gets to the third minute do 10 pushups, and so on. So basically you are doing 10 pushups and resting within each minute for 15 minutes. Once you get good at that do elevated pushups. That is a good workout to get you started.
One of the Crossfit workouts I like to do is 400 meter run, 50 situps, and 50 back extensions and do that for 5 rounds.