Q: how can i download itunes

There are three ways to delete all of your data.

On the Phone:

Settings --> General --> Reset --> Erase All Content and Settings

Through iTunes:

Connect Device.

Click on the iPhone icon in the upper left area of the window.

Under the summary tab, you should see an option to restore the iPhone in the middle. Click it.

Through iCloud:

Sign into iCloud.com on a computer, or the Find My iPhone app on an iPhone.

Select the device you want to erase.

Click erase device.

You asked two questions, one on the title and one on the body.

http://www.apple.com/itunes/download/

Erase all content and settings on your iPhone, iPad, iPod touch, or Apple Watch - Apple Support