I have to make a public kill in Whiterun and it has to be a high profile target, then I have to plant incriminating evidence of their body...... can someone please help with this? I'm trying to kill someone who won't effect the game too much

The quest may be completed by killing anyone in the city, not merely a high profile target. You will still receive a reward, even if you fail the optional objectives. If you merely wish to complete the quest, you can kill a guard. Or if you wish to avoid a murder charge, you can lure a hostile actor from outside the city near it and then kill them.

Just kill a guard, unseen and plant the letter. The optional components are just that optional. So my advice is don't bother trying its not worth it.