First you install the pre-requisites:
curl -sL https://deb.nodesource.com/setup_10.x | sudo -E bash -
apt-get install -y nodejs make g++
npm i ionic -g
Then, you need to run
npm install
After that, to preview in your browser simply run:
$ ionic serve
To preview in the emulator, run:
$ ionic cordova platform add ios
$ ionic cordova run ios
Substitute ios for android if not on a Mac.