I lost my mother when I was 19 to cardiac arrest brought on by heavy alcohol addiction. I know the pain you're going through and I understand it very well.
The first thing you should know that grief impacts everyone differently and it doesn't matter if the death was expected or sudden. All loss of life is tragic and expressions of that loss can take the look of many, many different things. Don't always go by the whole "stages of grief" and whatever the rest are rules, because there are no rules for agony like that. No matter how much time you have left with your step-father, it will never be enough and it will never feel like enough. I'm sorry, I hear your pain, and I empathize with it.
I recommend looking into getting a grief counselor. Most of the time their services are free as they work with the family members of those who are in palliative care, but you can always follow up with them for more complex therapies. There's groups available for these situations. Next, I recommend reading as much literature as you can about loss. It will be extremely hard and a lot of it might not make any sense to you, but I implore you that it's an important step for self care. It has been ten years since my mom died and I'm still dealing with how her death impacts me to this day. You as well, like myself, will probably deal with it for the rest of your life. And sometimes you're going to feel like you're fucking crazy.
But there is a life for you beyond grief and tragedy. You are here and you are a survivor, even if it doesn't always feel like it. I don't know how much more time you have with him but be with him. Share these last moments with him. Take it one day at a time. Losing a parent, guardian, or loved one requires you to make a new normal for yourself. It's hard and it hurts but you too will find a new normal without him.
I'm so sorry this is happening to you. If you need any advice about the more practical steps when it comes to funerals I can make some recommendations about that too. Practice self care, forgive yourself, and breathe.