Format and fresh install
If you just want a fresh start with your phone, you don't need to reinstall the OS at all, because OS files and user apps and data are stored separately. In normal use, the OS partition is read-only. All you need to do is a factory reset. This wipes all user data and apps, but doesn't usually touch the SD card. The process to follow is slightly different for different phones, but there's usually two ways: either reboot into recovery mode, which has a "factory reset" menu item, or from Settings → Backup &amp; Reset.