If I was to erase my phone will I loose my contacts and photos and other data

If the only place you stored them is on the phone then yes you will.

If you have it ALL synced to iCloud (eg iCloud Photos, iCloud Drive, and contacts toggled on in iCloud) or sync'd on another cloud system or email account then no.

You are advised to back up to a computer and to iCloud before wiping the phone this way.