How can my Dad get his US Citizenship?

well, you really should consult a good immigration attorney before doing anything. the marijuana conviction is very problematic, depending on just how much grass was involved. if he came illegally to begin with, he won't be able to adjust status in the US and would have to leave to apply for an immigrant visa. unless your mother is either a citizen or a green card holder, he won't be able to file a waiver of his illegal stay in the US and would be stuck back in his home country for 10 years, which doesn't even take into consideration the drug conviction. So, don't start anything you aren't prepared to finish and try to make sure there's at least the possibility of a happy outcome.