It will suggest you some murder targets the AI would choose if it were you. Still no luck? Then just go to the Intrigue window and click on 'Click to choose a plot'. A claim is usually due to a short family connection to the actual title holder which means they might be close to it in line of inheritance. When you are looking at people among your vassals who have a good chance to gain a title through a convenient death, check out which one have claims. When it is a man, you can try to invite him to your court. When it is a woman, you can try to marry her to one of your vassals (you might have to make her a widow first). Then you just need to make that person your vassal. When you have a good understanding of the different inheritance rules, going through the family tree of the title holder is another way to find them. Unfortunately the tooltip does not allow you to click on any of the names, so you need to type them into the character finder (shortcut. You can do that by hovering on the shield-icon of the title on most GUI windows.
When you are interested in gaining a specific title through murder, find out who is in line of inheritance of it. Vyndicu's answer focuses on plot power, so I will focus on finding good targets.
