mainline kernel images: audio does not work on all devices, Bluetooth is unstable.
Make sure you have a good & reliable SD card and a proper power supply. Archives can be uncompressed with 7-Zip on Windows, Keka on OS X and 7z on Linux (apt-get install p7zip-full). RAW images can be written with Etcher (all OS).
How to boot?
Insert SD card into a slot and power the board. First boot takes around 3 minutes then it might reboot and you will need to wait another one minute to login. This delay is because system creates 128Mb emergency SWAP and expand SD card to it's full capacity. Worst case scenario boot (with DHCP) takes up to 35 seconds.
How to login?
Login as root on HDMI / serial console or via SSH and use password 1234. You will be prompted to change this password at first login. Next you will be asked to create a normal user account that is sudo enabled (beware of default QWERTY keyboard settings at this stage).
How to connect to your router via WIFI?
Required condition: a board with onboard or supported 3rd party wireless adapter on USB
Gigabit ethernet transfer rate is around 50% of its theoretical max rate (internal chip bus limitation)