This is an excellent video about how to build your upper body strength with variations on the push-up. It spans the full spectrum from easy to hard. Everybody can find a good place to start and an on-going challenge here.
It accompanied an article at Mercola.com. The article has a dumb title, but the content is very good.
Push-ups are a near perfect exercise. No matter how strong you are or young or old, you can do them anywhere, anytime, and they require no special equipment. If you never lifted a Olympic weight or kettlebell in your entire life and just did lots (say 100 total in sets of whatever number make you hurt a little) of push-ups every day, you'd be pretty strong from head to toe and get a decent cardio workout.
Runners and bikers sometimes overlook the importance of upper body strength. Remember that if you fall, your arms and shoulders are the first line of defense for your skeleton.
Don't think, do them.