If using stock Android, go to Settings > About phone > Build number. On a Samsung Galaxy device, go to Settings > About device > Build number. On an HTC device, go to Settings > About > Software information > More > Build number. On an LG device, go to Settings > About phone > Software info > Build number.
Tap Build number seven times. After the first few taps, you should see the steps counting down until you unlock the developer options.
Once activated, you will see a message that reads, You are now a developer!
Go back to the Settings, where you’ll find a Developer Options entry in the menu.
The developer options in Android allow you to carry out a host of actions, but they’re mostly designed for developers to debug their apps and tweak different system behaviors. I would advise against tweaking any settings without first understanding what they do. A Google search of any function you are not sure of would do the trick.