Start by drawing a relationship map of all the characters. Give each character a letter or short abbreviation so you can also make an indexed table with rows corresponding to these characters. Give the table three blank columns. Label the columns "motive", "means" and "opportunity".
motive means opportunity Char A Char B Char C . . .
In a good mystery, there will be two people who have all three of those things; they will be the initial suspect and (twist!) the murderer. Everyone else will be crossed off the list over the course of the investigation as they are discovered to be LACKING one or more of those three things. The murderer will be winnowed out by one final clue late in the third act, often just preceding a climactic fight or attempted flight scene.