My iphone is syncing a random strangers contacts and deleting mine.

Your phone shouldnt be syncing others contacts unless you and someone else have both used the same computer (itunes) to install and setup your phone..i.e if you and your partner share the same computer you may sync his contacts unless logged into seperate itunes accounts.
(I may be wrong but thats what happened to my mum and dad in the past) I hope that helps.

Stop syncing someone else's iCloud account to your device.

If you are sharing an Apple ID for someone, for the purpose of iTunes media or apps, do not use that same Apple ID for iCloud.

Change your AppleID password.