This is going to be a pretty simple post. Just giving a basic guide of what I had to do to install oh-my-zsh.
brew cask install iterm2
(it should be mentioned you need to install homebrew first)
brew install zsh zsh-completions
Then simply check
echo $SHELL if that doesnt have some form of zsh in it you simply need to run this command to change the shell path
chsh -s $(which zsh) and restart iTerm. You can them simply change the theme by opening
~/.zshrc and changing the ZSH_THEME variable to whatever theme name you would like to use.