Sometimes, you and your Android phone just don't get along: You want to watch a video and it won't stop pinging you with notifications. You have hundreds of apps installed on your phone and trying to figure out which one is incessantly bothering you would take too much time. In these situations, the scorched-Earth method works best—provided you don't mind losing all your personal data.
To reset your Android phone back to its pristine, fresh-from-the-factory state, simply head on over to your phone's Settings app and look for the Backup and Restore menu. Tap on it, and you should see an option labeled Factory data reset. Selecting this option will pop up a window asking you to confirm your selection.
At this point, note that if you choose to proceed, you'll lose all data on your phone, including your apps, photos, music, and any contacts that aren't backed up to Gmail's servers. If you're okay with starting over, hit the reset phone button and your phone will begin the process of resetting itself.
Resetting your phone can take anywhere between 5 and 10 minutes, depending on how much stuff you had on it. Some phones won't let you reset them unless you have over a certain percentage of battery, so it's best to perform this operation while the phone is plugged in and charging. Once the phone is done purging itself of all your data and apps, you're phone will be just like new—software-wise, anyway—and you'll be prompted to go through the setup process once more.