How to delete everything on my iPhone 4?

I used my moms old iPhone 4 for a while. Now she wants it back but I don't want to give it her with all my information, my notes, my contacts. I was wondering if there's a way you can go in settings and it will delete it all for you. If there's please can you tell me how to find it and what it does thank you:-)

Yes it's easy
Go into settings
General
Reset
Reset all settings

Reset iPhone to Factory Settings with iPhone Only
Warning: This will removal all data, music, photos, settings, literally everything, from the iPhone. Back up before beginning if you need any of your personal data again:

Launch "Settings" and tap on "General"
Scroll to the bottom of General and tap on "Reset"
Tap on "Erase All Content and Settings"
Enter the iPhone passcode if one is set, and confirm the reset by tapping on "Erase iPhone"

Resetting the iPhone on device can take a while depending on the model, the warning dialogue will give you a rough estimate but it can range from a few minutes to over an hour to restore the device completely. During this time the phone is otherwise inoperable, as everything is be cleared off.

Once completed, the device will reboot and show the familiar new setup screen. You'll either want to leave the iPhone in it's factory new state for a new owner to configure, complete the setup as a brand new device yourself, or restore from a backup if you initiated the factory reset for troubleshooting purposes.

Calendar Removal from iPhone
1. Press the "Home" button on your iPhone and tap the "Settings" icon on your screen.
2. Tap the "Mail, Contacts, Calendars" tab on the screen and tap the account in the "Accounts" section that has the calendar you wish to delete.
3. Slide your finger across the "Calendars" switch so that it reads "OFF."
4. Tap the "Delete from My iPhone" button to erase all events on your iPhone. Calendar Removal from iTunes
5. Click the "iTunes" icon on your dock for Macintosh computers, or click "Start," "All Programs," "Apple" and "iTunes" to open iTunes on your PC.
6. Connect your iPhone to your computer by plugging the USB cable into both devices.
7. Click the iPhone tab in the iTunes sidebar and click the "Info" tab.
8. Remove all of the calendars on your iPhone by unchecking the box next to "Sync iCal calendars" or uncheck the box next to a specific calendar to erase it. Click "Apply" to complete the removal.

Go to your settings and erase all content.