I was in a similar situation when starting out. I couldn't do a single pull up. Heck, I couldn't even pull myself up an inch. So I substituted lat pull downs using elastic bands. I would hook the elastic bands at the top of my door, place an ottoman next to the door to give me leverage so I wouldn't be dragging myself along the floor instead of pulling the bands down. I would substitute 3 lat pull downs for each pull up I was supposed to do. So, if I was supposed to do 3 pull ups, I would do 9 lat pull downs. Eventually you will be strong enough to do pull ups. I can do about 12-14 non-kipping pull ups in a row now. Also you state that you don't have anywhere to put a pull up bar. Can't you just use a doorway pull up bar? That is what I use for indoors. I have a pull up tower that I am placing outside to do outdoor workouts from spring to fall when the weather is permitting.