Can I help my step dad get a green card?

In order for you to file on behalf of your stepdad, you have to be over the age of 21. The rule provides that in order for a United States Citizen to petition on behalf of a parent (which includes a stepparent relationship), the petitioner must be over 21 years of age. Do you have other siblings that may be able to petition for him? You should speak with immigration counsel to determine whether you have any other options.

As long as the marriage took place before you turned 18, he is your "dad" for immigration purpose. However, you need to be 21 (and American citizen-You did not mentioned your status) in order to start the process for him. Other important admissibility issues to consider: Does he have a legal entry (with passport and / or visa and or / border crossing card)? Does he have any criminal issue(s)? Any fraudulent immigration paperwork submitted in the past? Any deportation? The list can go on and on. Just make sure to check all the admissibility issues with an attorney once you turn 21.