Start from wall pushups and go to the normal ones as you build strength did 73 straight max once :D
Should I first lift dumbbells for strength or any other methods u prefer for gaining strength??
Well yeah bicep Curls will increase your arm strength and do assisted pullups bring yourself down real slow and go up with someone's help. Proper diet with good balanced diet will surely help you :)
I've started doing kneeling pushups, but you could try wall pushups as well. Simple and effective. I've started doing 3 sets of 10 reps and at first it has been hard to complete the excersise but after a while I was able to do a few normal pushups. I've been suggested to have a 30-60 sec rest between sets, I'm not sure why is that... Good luck :)