This might be the most important step of all. Since phones and tablets are easily lost or stolen, you want to make sure it's as protected as possible in case anyone else gets your device.
First, head to your device's settings by hitting the menu button (usually the leftmost one that looks like a simple menu). Select Settings, then scroll down to and tap Security.
In that screen, you can set a PIN code, pattern, or, depending on your device and Android version, another means of locking the phone or tablet (e.g., face unlock or password). A long, multi-character password has the highest security, but if that's too daunting to enter every single time your screen locks, at least set up a long PIN.
Depending on your device and Android version, you may have other security options, such as encrypting the entire device (important if you're using your phone or tablet for work) and locking the SIM card.
If you have the option to enter owner information, definitely set that up (your name and an alternate phone number), just in case you lose your phone and a Good Samaritan finds it.
Finally, you'll want to set up remote wipe as soon as possible. This allows you to erase all the data on the phone or tablet from afar, in case it gets lost or stolen.
Set up wireless connectivity
At this point, you'll probably also want to connect to your Wi-Fi network. Leaving Wi-Fi on all the time isn't a great idea for your mobile device's battery life, but when you're at home or at a known wireless network, it is best to use Wi-Fi.
Head to Settings again from the menu button, then go to Wireless & Networks and tap Wi-Fi. Enable Wi-Fi and tap the name of your wireless network. Enter the network password (if any) and you're ready to roll.