your knees are going to be sore the more work you put on them. The thing is the joint get stronger as your muscles do too. The stronger the muscles around the joint, the less stress the joint takes.
Variations of push ups will help your chest for at home workouts. Wide stances, standard, and diamond will mostly hit all parts of your chest. The thing is, if you want to put on some real size and mass there. You will need to eventually start lifting.