this is difficult to write an obituary for a character in Hamlet because I only seen the movie of hamlet yet I have to write about them as if they where people that I have known all my life.
Just curious - Was it your assignment to read the play, or to just watch the movie? Your question mentioned that you had only watched the movie...I would think that writing an obituary for one of the characters would be a whole lot easier if you had spent time reading the play. And which movie version did you see? I can think of 4 (Olivier, Branagh, Gibson, Ethan Hawke), and only choosing one to watch could definitely make a difference in how you view and understand the characters.
Please read the play, even if that wasn't your assignment. Use the eNotes links to help you understand. But a movie version isn't ever going to give you all the information and great stuff you can get from a read of Shakespeare's own words.
First, you need to choose a character. For instance, Ophelia. Next, you need to find qualities that define his/her character-physcial description, motives, things they say, believe, do which help you to "know" them as real people. It would help if you skim through the play and find the places where the character you've chosen is mentioned by other characters or is on stage performing. Just as in real life, we know a character in a play by how he dresses, what he says, what he does, how he reacts to others, and what others say about him. Make a list of all the things you can find about the character you choose, and then from that, write your obit.
For Ophelia, we know she is young, beautiful, of noble birth, in love with Hamlet, a dutiful daughter, loyal sister, and she loves flowers. I would probably put a twist on things and write her obit with flowers...each one has a different meaning just as she gave them for "remembrance," etc.
Check the link here for a few places to look for the meanings of flowers, or go back to enotes and check the character analysis to choose another character whom you like better for your obit.