There are a few different methods for adding a reminder to Google Now. You can use your voice or enter it manually.
To create a reminder by voice, launch Google Now and say "Google" or tap on the mic icon. Once you hear the beep to alert you that Now is listening, start the command with "Remind Me" and add in what you're afraid of forgetting. You'll also need to add a time or location for when Now should remind you. You can set a specific time, or a general time frame, such as afternoon or evening.
When adding a reminder by voice, Now will present a card to you populated with the information you requested, allowing you to verify it's correct. Tap on the "Set Reminder" bar to save it to your account.
To add a reminder by hand, launch Google Now and scroll to the bottom where you'll find the menu icon and tap on it. Select "Set Reminder" from the list of options. You'll then need to enter a title and select the alert trigger (time or location).
When adding a location to your reminders in Now, it does something pretty amazing. When you need to be reminded to grab some milk at your local grocery store, just say the name of the store instead of the address. Google Now will find the closest location and use it as a trigger for the reminder.
You can't do with with Siri unless the address is saved under a contact in your address book.
Unfortunately, you can only access reminders in Google Now, and you can't edit existing reminders. To access currently set reminders, you have to launch the menu in Now, choose Settings > My Stuff > Reminders. You'll see a list of your current and past reminders. Tapping on a reminder will give you the option to delete it, and nothing else. If you need to change anything about the reminder, you'll have to delete the current one and add a new one.