Run this: sudo apt-get install zsh
Verify where zsh is by typing whereis zsh.
Run chsh and it'll ask you to type in your password and to enter the new value. If you've verified that zsh is available on /bin/zsh, type it in.
Warning: Be very careful when typing the path, as an invalid path given to chsh could cause a lot of problems.
Final step: Re-login
I switched yesterday to zsh, all I had to do, was
sudo apt-get install zsh
chsh -s $(which zsh)
Close all your terminals afterwards and you are done.
You can customize your shell further with Oh My Zsh.
These shells do not use the same configuration files. Normally the PATH environment variable is set in /etc/profile and ~/.profile which are files read by bash (and dash) but by neither of fish or zsh.
But you can manually execute your profile files (this sets PATH and does other things as well)
For fish you can use the script provided here: https://...