Use adb or a terminal app.
su
rm /system/bin/su
rm /system/xbin/su
rm /system/bin/busybox
rm /system/sbin/busybox
Now go to Settings/Application Manager and uninstall Superu or SuperUser or whatever app pops up to ask for root permission for a rooted app.
(If you used a terminal app, uninstall it.)
Then Backup and Reset/Factory reset.
The phone should be about the way it came out of the box.
This is assuming that you didn't use any rooted apps to change anything on the phone. If you did, you'll have to change them all back. And you should uninstall any apps that need root - they don't belong on a non-rooted phone.
Or you could just do a factory reset. That wipes out everything you added as a user. The phone will probably still be rooted, but root checking apps will report that it's not. And all your personal data and the apps you installed will be gone.