I cannot do a single push up?

Focus with cardio. for 6'6" 350lbs. you are either muscular or fat. In this case probably the latter. If you want to start doing push up, you should do "girl" or knee push ups. You can do negative push up as well. Try to go down slowly and go up slowly. Do as many as you can three times during your workout everyday and you should be up to 25 pretty quickly. It's mainly consistency that you need to worry about. You can also do pull downs (pullup but going slowly down) have a partner help you up and slowly go down.