you don't get the strength to do one arm push ups by doing two arm push ups. There is a technique to it and if you do a search through the past threads it should bring it up. You need to approach it like a power lifter, low reps a couple of times a day should do the trick.

But to begin with. Do a 1 arm press up against the wall, take your feet shoulder width apart, your spare arm loosely on your lower back. You need to focus on not dipping the opposite shoulder to arm and your back straight as you lower, when you push up your arm should corkscrew a little. Now I figure for most of you the wall will be easy. So go lower on a window sill, then a table, side of the bath then the floor! whatever is around you.