How do you volunteer for a deployment while serving in the army NG or reserves?

Yes you can, but when you get to the Guard or Reserves you will need to come into the armory or office to do that. It's called Tour of Duty on the MOBCOP website.
https://mobcop.army.mil/
You can even get on it now at work to create your profile and learn how to use it while you are still active duty. But it is only available on the mil domain, you cannot access it from the outside network. So, once you are in the Guard/Reserves you are limited to searching and updating it from the closest armory to your house.