Open the Settings page by clicking on Settings in the upper right hand corner of your account, and select All Settings from the drop down menu.
In the left-hand menu click on My Settings.
Find the fields called "Default Event Reminders" or "Default Task Reminders". Then, setup your reminder schedule however you'd like it.
When your reminders are setup, click on Save Preferences to apply the changes.
Now, every time you create an event, this Default Reminder Schedule will be applied to the event for you.
Default event reminders can always be overridden on an individual event.