If you are not currently able to do 100 push ups then I suggest you start out by doing sets of push ups.
For example-- lets say you can only do 5 or 10 push ups... do as many as you can and then rest for 1 to 2 minutes then do another set of push ups.
Again as many as you can do. Write down your results. Do push ups every day. Do at least 5 sets of as many as you can do.
In less than 30 days you will be doing 100 push us with out much of a problem!
Keep track on paper of your results. Purchase a set of perfect push up its a great device and takes the strain off your wrist and puts it on the muscles.
http://hundredpushups.com caters for a wide range of strength levels and everyone I've sent this to has found it very effective. Just gives you reps and sets that consume little time.