Read more @ LifeHacker.
Google Operating System explained how this works. All you have to go is visit Google and, for reminders, type add reminder or create reminder into the search bar, and the interface will change to let you fill in the details. For reminders, you'll get a field asking what you'd like to be reminded of, when, and where. You can even click to have Google Now remind you on your mobile device. If you type in more details from the start, like "add reminder to call Whitson tomorrow," the fields for when and what will fill themselves in automatically.