you can't really develop the muscles after a while if you only ever do bodyweight exercises. but to start you can do pushups, this works a lot of the upper body, not just the pecs. if you have dumbells of a decent weight then you can do flyes as well. once these become easy, either do more weight for the dumbells or more pushups in a row.
these are pretty much the only pecs-work things i can think of right now, don't forget to work abs and back as well with crunches/situps and bridges