Examine the themes of your script. If it deals with death you do not want to have sunshine or rainbows in the title.
Write the plot of the movie on a piece of paper. A good title (I use the word "good" because many movie titles are terrible even if the movie is good) will tell the story of the movie in a few words.
Read your script. There may be some metaphorical dialogue, or a repeated line that makes sense as the title.
Are there any characters that have a cool name? We get so used to titles like "Jerry Maguire" that we forget it is merely a character's name.
Is your movie about a specific person or event? If so, name it after the event of person. Nobody has any confusion over what "Malcolm X", "World Trade Center", or "Pearl Harbor" is about.
Ask around. Let a few people read the script and test some titles on them. If they know the plot, they will also know if the title gets the story across.
Remember that a good title gets the plot across. There are plenty of bad titles, but a good one will attract viewers and get the plot across.