How to damage everyone who touches a part?

Start off by using the Touched event.

script.Parent.Touched:connect(function(Hit)
    return Hit
end)

This is a basic Touched event that fires when touched, The Hit parameter is a object that Interacts with the Part. Next, damage the player, but we'll check if Hit is a Player, by accessing the player and search for a Humanoid.

script.Parent.Touched:connect(function(Hit)
    if Hit.Parent:FindFirstChild("Humanoid") then
        return Hit
    end
end)

Now the final step, taking damage, You can take damage by using the TakeDamage method.

Damage_Taken = 50

script.Parent.Touched:connect(function(Hit)
    if Hit.Parent:FindFirstChild("Humanoid") then
        Hit.Parent.Humanoid:TakeDamage(50)
    end
end)