Vyndicu's answer focuses on plot power, so I will focus on finding good targets.
When you are interested in gaining a specific title through murder, find out who is in line of inheritance of it. You can do that by hovering on the shield-icon of the title on most GUI windows. 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 .). 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.
Then you just need to make that person your vassal. 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). When it is a man, you can try to invite him to your court. Both require them to not be a ruler.
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. 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.
Still no luck? Then just go to the Intrigue window and click on "Click to choose a plot". It will suggest you some murder targets the AI would choose if it were you. The first in the list will usually be people your character just has a very negative opinion about, but there might also be some good suggestions in the list of people whose deaths would benefit you politically.
No good target in reach? Well, then you need to make one.
- Marry someone from your court to someone who is in line of succession of an interesting title. When that inheritor is a man, make sure it's a matrilinear marriage (this usually will only be accepted when the man is rather far down in inheritance and the bride is a really high-level noble).
- Wait until they have a kid
- Give the kid a landed title in your realm
- Have unfortunate accidents happen to everyone higher in the inheritance chain so the parent gains the title
- When the parent dies (naturally or due to your help), the kid will gain the title and unless the title is on a higher level than your primary title, the kid will stay your vassal.