i used to have iphone4 which i gave to someone when i got 4s, the thing is her contacts constantly comes to my iphone4s and changes my contacts to hers. why this is happening please

You should use separate Apple ID's for iCloud, iMessage and FaceTime. These services are separate from iTunes.

Create a new Apple ID > https://appleid.apple.com/cgi-bin/WebObjects/MyAppleId.woa/ -- Note: You need a verified email address

On one of the devices go:

Settings > iCloud > (Scroll Down) Delete Account > Sign in with the new Apple ID -- Note: This only affects the device

Settings > Messages > Send and Received > Apple ID > Sign Out > Sign in with the new Apple ID

Settings > FaceTime > Apple ID > Sign Out > Sign in with the new Apple ID

This will separate your iCloud content (i.e. Contacts, Notes, Reminders etc.), Messages and FaceTime.

You both can continue to use the same Apple ID for iTunes and share purchased Apps, Music and Movies