If you are unable to do a standard push up, try it this way. Get a medium sized beach ball; put it under your chest. Drop down and do a modified bounce on the ball. This will give you enough of a head start to finish the exercise. Once these are easy then move into the regular push ups.
You can also do these with elastic material fastened above and then slipped around your chest. Do not allow the elastic aid to wrap around your throat and choking off your air supply.
As you drop down toward the floor or ball the material helps unload some of your bodyweight making it easier to push back up to the top.