How must a man start focusing on producing healthy sperm?

You cannot. Staying fit and avoiding drugs and alcohol may improve your sperm quality (i.e., your ability to become a father), but nothing you can do can change the genes that your sperms carry. If it is your goal to be a father, then think instead about how you can be the very best father to your future child. It is perhaps a man's greatest trial and possibly greatest triumph. Good luck.