How would I get where the player is clicking?

Put this code in a local script inside a tool. This code will place a block with the center of the block where the mouse clicked. You can change the size of the block and other variables for it in the part() function. I don't know how much this helped, but if you have questions feel free to add me on skype and ask me there. mstrgmrs is my skype.

local tool = script.Parent;
local player = game.Players.LocalPlayer;
local mouse = game.Players.LocalPlayer:GetMouse();
 
function part(pos)
	local part = Instance.new("Part", workspace);
	part.FormFactor = Enum.FormFactor.Custom;
	part.Material = Enum.Material.Neon;
	part.Size = Vector3.new(2, 1, 2);
	part.CFrame = pos;
	part.Rotation = Vector3.new(0, 0, 0);
	part.Anchored = true;
	part.CanCollide = false;
	part.Transparency = 0;
	return part;
end;

script.Parent.Equipped:connect(function()
	script.Parent.Activated:connect(function()
		part(mouse.Hit);
	end);
end);

This code here will make the block placed go to the nearest whole stud

local tool = script.Parent;
local player = game.Players.LocalPlayer;
local mouse = game.Players.LocalPlayer:GetMouse();
 
function part(pos)
	local part = Instance.new("Part", workspace);
	part.FormFactor = Enum.FormFactor.Custom;
	part.Material = Enum.Material.Neon;
	part.Size = Vector3.new(2, 1, 2);
	part.CFrame = CFrame.new(math.floor(pos.x), math.floor(pos.y), math.floor(pos.z));
	part.Rotation = Vector3.new(0, 0, 0);
	part.Anchored = true;
	part.CanCollide = false;
	part.Transparency = 0;
	return part;
end;

script.Parent.Equipped:connect(function()
	script.Parent.Activated:connect(function()
		part(mouse.Hit);
	end);
end);

The only difference between the two is the math.floor() for the position instead of the exact position.