I am very sorry to hear that your dad is so ill that you have been told to prepare for that he might die. Your question is very important and also very difficult to answer. How do you prepare for death -particularly when you have to prepare for your own dad to die?
There are many factors that will influence how people prepare for losing a parent. It can be things relating to how old you are, what your situation is with schools, exams etc., your relationship with your dad, how you communicate and support each other in the family, support from friends and others around you, whether your dad is able to still talk to you, whether you are witnessing your dad in pain and distress, whether your dad is at home, in hospital or in a hospice etc.
In a situation like this you are likely to feel lots of different emotions like sadness, anger, anxiety, loneliness, guilt etc. It is very important to talk about how you feel to people you trust so that you don't bottle up all your emotions. It is harder to cope with emotions if they keep building up inside you as opposed to dealing with them as they come. Of course, no matter how you much you talk about your emotions, nothing can take away the pain of what you're going through, but at least sharing a heavy burden with others are always better than carrying it alone.
I recommend that you have a look at the section on the website that's called "if your parent is dying". If I was to recommend one thing in preparing for death - it would probably be that you stop and think if there is anything particular you want to say or do for your parent at this time. This can make you feel better so that you don't blame yourself later on for not having said and done things you wanted to. Don't push yourself if you don't feel it's right for you though. And remember, there are so many ways of saying things and we don't always have to say things out loud. Some people find that they can express themselves best by writing a letter to their parent, or maybe a poem or maybe make a drawing. There can be other ways of showing your dad that you love him too, which you feel is more right for him and for you so have a think about this so that you don't regret things later on. If your dad is too ill and you can't talk to him or he can't read or otherwise communicate with you - then try and not feel guilty about things you wished you had said or done. A big hug can sometimes say more than many words so keep on showing how much you love him in the ways that you can. I'm sure your dad will know what you feel about him and how you desperately wished that this wasn't happening.
It sounds like you also have good support from nurses and others around you so you could also ask them if there's anything they can think of as they will know both your dad and you and the whole situation much better.
There will of course also be many practical things to think about if someone is going to die, but adults will take responsibility for this and involve you where this is appropriate and right for you.
Finally, there will be many things in life that will change in the future if your dad doesn't make it and at some point you will feel the need to prepare for that. However, in a situation like yours, it may be best to take a day at the time and concentrate on what is best for you each day and get through this very difficult time that you're going through right now.
Thinking about you!