Just start the Reminders app, tap either the + symbol or a blank line and then type in the information you want to be reminded of. That's it. You will NOT be notified unless you specify a time to be notified.
If you have an iPhone 4S, you can tell Siri to remind you. For example: "Siri, remind me to buy a gallon of milk". When it asks, "When would you like me to set it?", reply with, "no time".