This is a very difficult question. First of all it makes me sad to know that you are probably asking because you are expecting that someone very very close to you is going to die in the not too distant future. It is also a difficult question because people react very differently after having lost a loved one so it's impossible to say anything specific about how you might feel in your situation.
It's important to know that there is no right or wrong way to feel when someone has died and there is no right or wrong way to react or to grieve. There is so much to say in order to answer your question in the best possible way so I think the best thing for you in order to get a full answer would be to read the section on this website called "If your parent has died" were we have written about common emotions and reactions after a parent has died. I think this section will be relevant to you even if it is not your parent you are thinking of but someone else who is very close to you.
Hopefully this will give you some insight into what it may be like after having lost a very much loved person.
Thinking about you and hope that you have good support from other people close to you. Please get back in touch with us if you want to talk more about things that are on your mind.