It's not possible to boot from external devices on a Chromebook as they use a firmware NOT a BIOS. It is possible to install it in a "chroot", where you use a directory as your filesystem and run it alongside ChromeOS. This is probably the easiest as it doesn't require you to partition or otherwise modify your Chromebook.
It IS possible to run SeaBIOS on at least some Chromebooks which make it function more like a standard laptop however that installation process isn't as simple as chrooting and offers minimal benefits if you just want a Linux environment to mess around with.
the setup process is relatively simple and you should be able to find a guide through a quick Google search.