Outlook isn't just about e-mail. It's also about organizing your time, which you do in the calendar. When you create or open an item in your calendar, you'll see that the Ribbon shows groups and commands appropriate for helping you manage your time.
When you create any type of calendar entry, a reminder is set automatically. To change the reminder time for an appointment, do this:
On the Appointment
tab, click the arrow to open the Reminder
list and then select a time.
Once you've made a change, click Save & Close
on the far left of the Ribbon.
Note The way that you set the standard reminder time for all appointments has not changed. You still do that by clicking Options on the Tools menu in the main Outlook window, and then setting the default time on the Preferences tab, under Calendar.