In Apple Remote Desktop I see my computers but they are offline. How can I put them on line so I can control the computer
You have to enable remote desktop sharing on the target computers.
Apple has an excellent kickstart resource to enable this setting remotely. See this Apple Support document and also if your network supports it, you can send a wake command if the network is active while the machine is sleeping. That last item won’t start a powered down machine or reach across routers, only machines oN the same subnet can be woken.