Secure your device (before it is lost or stolen)
Set a PIN or password
The most important security measure on an Android Smartphone is to set an PIN or password.
An unprotected device is directly accessible to every one if you have lost it while it was running.
For a the length of the PIN/password at least five or better six digits are recommended.
A pattern lock is also possible but not recommended as it is easier to crack and in case of an broken display it will will prevent you from accessing your device as it can't be solved with an attached keyboard.
Set or keep the PIN for the SIM
If your SIM allows for unlimited calls and does not have a PIN, someone might use the SIM even if he/she cannot use the phone.
So it's better to keep the default PIN, or set one.
You would have to unlock the SIM once after powering on the phone.
Disadvantage: You'll have to remember it (Only three attempts allowed).
Do not unlock bootloader or root the device
From the perspective of an attacker that have found or stolen your device an open bootloader simplifies a lot of attacks. Hence the general rule for secure devices is not to unlock the bootloader or root your device.
Know the device IMEI
If your device has been stolen you may want to complain this to the police. In such a case you need some device details like Phone brand and model number and the most important the device IMEI (most mobile network operators have IMEI blacklists of stolen devices). Usually the IMEI is printed on the boy the device is shipped in. Hence as long as you keep this box everything is fine. If not you can use your phone to display the IMEI by entering
*#06# in to the Telephone/dialer app. After entering the last dash immediately a dialog show pops up showing your IMEI.
After the device is gone
To say the truth if your device has been stolen and the thief is not an idiot your device is gone. No GPS/cell tower/Wifi location technology or whatever can locate the device or send back the location it the device is in a electrostatic shielded bag. Hence if your device has been stolen and you were unprepared it is now too late.
If you just lost your device or the thief is a complete idiot you may be able to use one of the location trackers. Regular Android devices that are "Google enabled" (Google Play Services and Google PlayStore installed and equipped with an active Google account) can simply use the Google Find My Device functionality.
Note that in case you have enabled two-factor authentication on your Google account make sure you have some backup codes in place. Because if logging into your Google account requires you to have access to your (now lost/stolen) Android phone you can't login and hence not locate your device.
I strongly recommend to test Find My Device while you still have access to your device just to ensure the functionality is really working.