Ah . . . well. Evidence, mostly. The deceased's body, the crime scene, the murder weapon . . . all of that is really important. And by finding other things connected to the crime, you can piece together what may have happened. Witnesses are helpful too - anyone who may have seen what happened before or after the crime, their testimonies are also evidence.
It's kind of like a big puzzle you have to piece together. And once you've got all the pieces . . . you'll find the suspect.