How do I make a part always look towards a player?

You would set the CFrame of the part to face the player (See: http://wiki.roblox.com/index.php?title=CFrame#CFrame.new)

CFrame.new(pos, lookAt) would create the CFrame at the position pos facing the position lookAt

If you want it to "always" face a player, you would update it in a loop or bind it to the render step.