How do I make a brick that kills you?

Basically I need a brick that kills you when touched. Tell what the code is like for example "if (touch) bla bla bla"

You shall utilize the BasePart.Touched event.

part.Touched:Connect(function(touchingPart)
	if touchingPart.Parent:FindFirstChild("Humanoid") then
		touchingPart.Parent.Humanoid.Health = 0
	end
end)

You'll use the following script:

part.Touched:Connect(function(hit)
if hit.Parent:FindFirstChild("Humanoid") then
		hit.Parent.Humanoid.Health = 0
	end
end)

But that's quite confusing, let's break it down.

part.Touched:Connect(function(hit)

In this section, we tell the part to connect to a function using "hit" as an argument. If you wonder what "hit" is, try doing:

part.Touched:Connect(function(hit)
	print(hit)
end)

It should print a body part of the humanoid, and every body part is parented to the player's character.

if hit.Parent:FindFirstChild("Humanoid") then

This line of code will check if the Humanoid actually exists, preventing errors if something like a tool or a flying part touches the other part.

hit.Parent.Humanoid.Health = 0

This is the last thing the script will do before ending, it's going to get the body part parent (Character) and refer to the humanoid's property "Health". After that, the script is going to set the health to 0.