Why does an alarm constantly ring and the home screen flash when I charge my iphone

That doesn't sound normal.

Are you connecting the iPhone to the wall with the accessories that were included in the box?

Maybe you are seeing some notifications and/or accessability alerts. Try putting the iPhone into Airplane Mode and connecting it to the wall adapter. If it doesn't happen after that, you could try turning Airplane mode back off and checking Settings > Notifications for possible apps creating the alerts.