How to Lock-on nearest player target?

I can't figure it out, can someone help me, please?

local uis = game:GetService("UserInputService")
local player = game.Players.LocalPlayer
local UiElements = game.ReplicatedStorage.UIElements
local char=player.Character or player.CharacterAdded:wait()
local focusing = false
Target = "placeholder"

uis.InputBegan:Connect(function(input)
    if (input.KeyCode == Enum.KeyCode.L) then
        focusing = not focusing     

        if (not focusing) then

            game.Workspace.CurrentCamera.CameraType = Enum.CameraType.Custom

        else
            local LockOnui = UiElements.Lockon:Clone()
            LockOnui.Parent=Target
            game.Workspace.CurrentCamera.CameraType = Enum.CameraType.Scriptable
        end
    end
    end)
while game:GetService("RunService").RenderStepped:Wait() do
    if (focusing) then
        game.Players.LocalPlayer.Character.HumanoidRootPart.CFrame = CFrame.new(game.Players.LocalPlayer.Character.HumanoidRootPart.Position,Vector3.new(Target.Position.X,game.Players.LocalPlayer.Character.HumanoidRootPart.Position.Y,Target.Position.Z))
        game.Workspace.CurrentCamera.CFrame = game.Players.LocalPlayer.Character.HumanoidRootPart.CFrame*CFrame.new(Vector3.new(3,2.5,5))

    elseif not focusing and Target:FindFirstChild("Lockon") then
        Target.Lockon:Destroy()
    end
end