I received a USB stick from an acquaintance. At least it looks like a USB stick. What preparation should I do before plugging it on my PC?
You could boot your computer to a LiveCD of some flavor of Linux, and then connect the USB stick. Any potential infections would most likely NOT infect your system. A further step would be to disconnect your hard drive first, THEN boot to the LiveCD. Disconnecting the drive first would guarantee that any potential infections would not reach your sensitive OS installation. If there was something on the USB stick, and it could infect the version of Linux you booted to, then just turning off the computer would remove the temporary OS from Ram and you would be clear.
This might sound overly paranoid, but if you really are worried, and you aren't necessarily confident that your AV suite can handle any potential infections that might be on the stick, this method only takes a few minutes to prep... and no installation whatsoever.