There are at least three ways to perform a system reset on your phone, but I'll just give you one - the simplest, and the recommended method if your phone is running well (i.e. no OS corruption problems, no hardware problems, etc.). The other two methods involves several button combinations and using a software to reinstall the OS.
Open your phone Settings > about > tap on "reset your phone". You will be asked if you really want to perform this action, since this action will erase everything custom about your phone.